We did 1000s of km's on dirt, rocks, dust, sand and wet clay. We did Central Oz, Cape Leveque, GRR, Tanami and Savannah Way plus all the Nat Parks and other bits and bobs we could find along the way to get away from other people
Our pre-trip prep consisted of a Stone Stomper, extended mud flaps on both the car and van. I slid the gal water tank guard right across to protect the tank fittings from impact, and inserted a folded gal filler piece at the other end where the tank was now exposed from moving the factory guard across. Bunnings sell small sheets of gal that fit perfectly into that exposed gap with no cutting or hardware required. A small strip of rubber over the sullage outlet was the only rubber I used because there wasnt any other way to protect that. Tucked up the water hoses that hang down like Aunty Gertys undies, and pushed them through the chassis where possible. (I had already re-plumbed most of our van with a mix of 25mm black poly pipe, and 40mm PVC with the sullage all running into a single sullage outlet. (The way it damn well should be) Pulled and pushed the elec brake wiring and tied it off to the top and rear of the suspension hardware out of the firing line (only using the slack and excess, i didnt re-wiring).
We had no sub floor damage all, and the only legacy is a distinct red tint to everything (which you just cant avoid unfortunately) and some serious pitting to the front face of the rear gal water tank guard.
I spent a bit of time rolling around under the van on a creeper with goo gun in hand and sealing all the gaps and all the joints I could find and also re-sealing the factory penetrations, along with the penetrations I did. I would say the most important, critical, and less obvious sealing was the joint between chassis and the black plastic wheel arch skirts. That area gets the biggest shellacking from the relentless dust. After 9mths on the road, much of it on dirt, for the first time we started to get some dust into the lower cabinets. We quickly found our sealer in that area had deteriorate. Stripping that back, resealing and we were back to no dust. I used thick course fish tank matting permanently attached for the door gas vent and I didn't seal off the fridge vents.