Yes must be a lot softer down here in the sand. 25 or 30 and you will be stuck real quick. 50km is about as quick as I get on the sand. Not that I get on the sand that much but when I do I look for something to climb so I prefer to play away from the water.
As for Rainbow Valley, for 22km you could throw caution to the wind, or un-hitch and have a look for maybe 5 or so km's and make a decision on what you find. Its your holiday and your van, you do whatever it takes to feel comfortable, prepared and confident that your doing it in your own time.
Theres no issue stopping every 10 minutes (or even 5 min) and having a look. Even after 1000's of km on crap roads I make a point of stopping every 50km or 30min to both break that tunnel vision, stretch my legs, check the car and van inside and out, and take a "me moment" to mentally note just how damn lucky I am (... and of course take another photo of the car and van, have a twinkle and grab a handful of my sons lollies from the van)
My personal view is if its a road, im going down it. If its a road how bad could it be. For 22km it youll only be on it for 30min tops even if it horrible. Tyre pressure for the car is pretty easy, you'll know because your in the car and your teeth will be rattling, the only consideration is less tooth rattle is better than more tooth rattle.
With the van is only ever going to be your very best guess because your not in the van, the tyre specs are way different to the car, the axles are closer together and the weight is different. There is nothing to compare to, and everyone else will also be guessing because they have the same criteria applied them. They may speak with an air of authority, but its safe to assume they have the same ability as you to not be in two places at once, and will also be guessing. I generaly used around 26 to 28 psi in the van for all tracks (that's only my personal best guess), the car around the same'ish.
The road surface will constantly be changing anyway so ideal tyre pressure and speed are very dynamic and will vary constantly minute by minute and metre by metre sometimes. You are at best only ever going to be right 10% of the time. I can guarantee you that the more time and km's you spend on secondary tracks of any condition, the less concerned you'll be concerned about it. More than a handful of time it was too hot, or for some other reason I just couldnt be bothered letting the air out .... and more than a few times I dreaded the airing back up so I saved myself the problem by not bothering to air down going in.
As for stuff moving arund in the cabinets, we use Corelle crockery, and china mugs and do nothing special with them in the cupboards etc. The Corelle lives all piled up in a stack in one cabinet as it would be at home. Dinner plates on the bottom, then bread plates, and bowls on top. No padding or additional anything. I dont know what they were doing whilst we were driving but they were always in one piece, and in exactly the same place Id left them.
For 22km Id also be aiming to take the motor home in as well, unless theres some specific reason not to. I haven't done Rainbow Valley, but it cant be any worse than where we've been. Inside the van is where most damage will occur but most of it is avoidable with a bit common sense, as opposed to feeling the need to re-engineering the van. Anything glass cant go near anything else glass. Rattle the drawers to their movement limits to see if you can work them open, because corrugations will do exactly that. Same with the fridge doors, over had doors. The Jayco ply is low grade and very thin. Drawer runner in particular are screwed on with tiny screws. I glued some thin scrap strips of MDF directly under the runners to take the pressure off the screws. Pack fridge and pantry items to minimise any movement, we use our pre-loved 1kg yogurt containers and just put stuff in them; bottle of oil with a bag a sugar, cordial with pasta, and bourbon with cotton wool, bubble wrap, tape, more bubble wrap, foam bumpers etc. Anything that can fall over will fall over so just pack it with something else.
We wrapped the griller tray in a old towel and put it back in, and ran a piece of tape on the stove glass lid from half way back over the front and around the griller handle to stop it bouncing up and down. Put the shower hand spray on the floor of the shower, because it will jump out off the wall mount possible snapping, and before it does that the metal hose will flap about scratching and discolour the shower wall. TV wrapped in a towel on our bunk bed never moved.
It you stand in you van in strategic spots with all your cabinets open and imagined what will happen to what you can see if the angry All Blacks were rocking the van from side to side trying to shake the mighty Wallabys out of the van, youll work out for yourself quite quickly what needs to be repacked, moved and protected.
Unfortunately you cant tell, for example, if a particular hinge has been over tightened and the screws are stripped in the timber; so you can never be sure if a door might fall off.
We found (or more intentionally put) ourselves on tracks where we found it impossible to even speak. Ill find a couple of pics of 1000's of km's of rubbish tracks we encountered and our van is still together, or back together. Ha