Regarding underbody damage, gravel roads arent a problem, its more the higher speed tracks with rocks.
As mentioned by
@MDS69, dust will potentially be your you biggest outside issue.
Like every van Jayco make there is limited consistency. With the same van model, one van will get no dust and the next will have more dust inside the van than there is on the road, and dust can absolutely ruin your holiday. Its a good idea to roll around under the van on a creeper (or crawl if your young enough) with a goo gun and go over every floor penetration and give another it layer. Sika roof and gutter will adhere to what Jayco use, dont be tempted to use something else because for example polyurethane wont adhere to what Jayco use. Area to pay particular attention is where the plastic wheel skirt meets the chassis, and anywhere where body moulding finishes where your likely to find massive gaps. Just fill any gap you see with roof and gutter, black under the van and white where it may catch someones eye.
Dust is like water, given enough of it, it will find its way through minute openings. Unless your planning lot of dirt roads, there's no need to go overboard, stick to the obvious.
Extra spare wheel, again only if your doing a lot of remote tracks. Your travelling with a mate with the same size spare so both of you have access to 2 spares anyway, plus an extra vehicle to take a 3rd puncture to the repairer for you. Your in a van, so you can camp wherever you stop. You can of course be $hit out of luck, but that can include 4 punctures between you and your mate, and where do you drawer the line. I limped our 3 ton van 220km west along the Gibb River Road back into Derby with only 3 wheels, and the 4th axle chained up to the chassis. I had an extra tyre case with me, every tool ever made, bead breaker, tyre change equip, spare parts and in that situation all of it was useless. Good tyres to start with, and your standard spare will serve you well. That goes for the car as well.
Stone Stomper priceless piece of kit for stone damage, and as a side benefit minimising dust. Dont leave home without a stone guard, and if you buy a stone guard your wasting your money if its not a Stone Stomper. They are annoyingly dirty things, but great in every other respect.
Diesels heaters. I have one (the truly peerless and awesome Eberspacher D2) and for us its a necessity. Diesel heaters are expensive and sometimes a mongrel to install. Central Oz will be freaken freezing overnight in July. If you no other use for a diesel heater, than save your money and take more jumpers. If you plan on doing a lot of cold weather of grid camping, a diesel is a must in my opinion. Roof top A/C units all have a minimum temp that they will heat, ours is around 4deg. Once the temp plummets below 4deg our A/C on heater mode just blows ambient air around. We use the diesel heater all the time in preference to the A/C, even in caravan parks
Water is surprisingly abundant. We have the 2 standard onboard tanks, 2 x 20ltr jerry can holders on the drawer bar and a 30ltr container in the back of the ute. In 14 months around Oz, including a lot of remote areas we never needed more than the vans 2 tanks. The extra was either a bonus for long showers, and we carried them empty. We use a Coleman HWS when free camping because the onboard pump uses way too much water for extended free camping. With the Coleman, and frugal use the three of use can get by easily with 25ltrs per day including all of us showering every day; thats 7 days on just the 2 tanks.
Fuel. I have a 150lts fuel tank and I still carry a 20ltr jerry in the back for no other reason than piece of mind. I never intended using it, but it provides a level of confidence. After 40,000km and 14mths I emptied into the car 3 weeks after we got home. But Id absolutely take it again. On any well trodden tourist route youll never "need" to carry fuel.
A full service on your car before you go and you don't need any extra filters and snorkel socks, just dont drive up the ar$e of your mate on the dirt roads. UHF radios are a must for convoy trips, so you don't need to see each other ..... (sometimes not seeing each other for a while is a good thing). None of these places are "remote", they all full of tourists with many locals on the same roads as you in their Commodore station wagons. Thats disappointing I know, but thats the reality. We almost destroyed our van getting into a particularly difficult and damaging camp area. The micro fell out, the oven fell out and table ended up at the other end of the van. We also broke every shelf in our Dometic fridge from corrugations ... always corrugations. No corrugations equals no damage. Our celebrations were short lived when we arrived only to find more caravans than the Melbourne Caravan and Camping Show.
Under body protection. I did minimal "protection" other than re routing plumbing to place as high and close to the floor as possible. I did replace all the idiotic looping water and waste pipes, the sullage in my van is now simply black garden poly pipe. I protected the water tank fittings by loosening the tank straps and sliding the factory tank guard across to cover those fittings. I then slipped a piece of folded gal into the other end. As mentioned by
@Drover, its not worth any risk to your water. Although where your going it wont be terminal, it will potentially ruin your day loosing all your water. A couple of cheap inline Bunnings taps fitted next to the factory tank selector that you can use to isolate the tanks is good cheap insurance. From factory, if you empty the front tank for example and park facing down hill, the front tank will fill from the rear tank. Its also handy with the taps because if one tank empties and the other is full and you cant isolate them, the pump sucking air from the empty one will stop water flow completely.
The piece of gal I slipped in with the factory tank guard is a standard off the shelf rectangle from Bunnings, not cutting required. The only rubber I used was under the sullage outlet because it is PVC was in the firing line of the vans rear tyre. I had a larger piece of rubber hanging behind the front water tank. Thats where my tyre case was strap to the chassis, and the rubber was only to reduce dirt and water filling up inside the in case I needed to use it. It didnt protect anything else
With a Stone Stomper its virtually impossible for anything to flick off the cars rear wheels, get past the vans front tank and hit anything. And on that note the front tank gal stone guard hasnt a single ding. From the vans drawer bar to the rear van water tank guard I have zero evidence of any significant stone strikes. If their was to be any obvious damage the first casualty would easily be the grease nipples on the leading edge of the suspension swing arms, particularly the plastic caps and they are all intact. (for the record the rear water tank guard was the only thing that took any type of hiding, but what a relentless hiding it took. Its prettiness has been compromised, but its structure and function remain intact)
We blew all 4 shocks, rattled a wheel off the van along with 3 studs and came home with stress fractures in the chassis. We ripped the spare wheel mount plate off the rear bumper arm, and drove the spare wheel up into the back wall of the van. We did Cape Leveque, the GRR, the Tananmi Trk and the Savannah Way along with many other tracks. We even did the northern first 100mtrs of the Canning Stock Route. The $hitty made and much maligned Jayco Expanda did very well by us despite my best effort to pummel it, so unless your going to flog the bejesus out of your van you should have no major issues doing very little work to it
Dirt or gravel formed tracks are not going to cause any more issues than bitumen. Whats more likely to go pear shaped is the internal van damage caused by corrugations if you start ploughing down every goat track you can find. Corrugations are the big problem; no corrugations equals no damage. Damage from corrugations is much more likely inside the van than outside. Dont focus too much of the outside, experience has shown me thats not where its all going to go pear shaped. Drawer runners, door hinges, microwave mounts table mounts, TV mounts and stuff not packed correctly are bigger more likely issues. Drawer runners coming off and drawers bouncing out and about in the van, bottles breaking, cordial bottle fracturing and leaking everywhere. My sons Xbox flew about 6 feet off the top bunk from the end of the bunk furthest away from the ladder. Amazingly Xbox's bounce surprisingly well. We collect large yogurt containers and put as much in the them in the pantry as possible and that stops stuff from banging together or falling over. Some other people on an NT freecamp track had their sink tap snapped off, and their over door glass shattered, splaying glass throughout the van. If you have a Thetford oven go and ask for the rubber grommet kit for the oven glass (thats the grommet kit that you only get if you ask)
Dust. Googun, creeper, roof and gutter and lots of it.
Im always envious of anyone heading away. Have a great trip
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