One important point that is easy to miss in how those expedition RV's are built - in addition to 4WD - is how they mount the coach box to the truck chassis. They use a 3-point mounting system so that any twisting or tipping of the truck frame doesn't get transmitted so as to try and twist the coach structure. The truck frame and coach structure must always be kept in a planar relationship to each other if 3-point coach mounting to the frame isn't used. Lack of this can tear apart the coach framing over time on rough surfaces. This is very important for extended off-road use to preserve the integrity of the coach structure.
We wanted a Class C we could take on desert roads occasionally for rock hounding, so as a poor man's approach to lack of 3-point coach mounting, we bought a 24 foot Class C on Winnebago's optional (for the size of the RV) E450 chassis. One of the reasons was to get the stiffest frame possible under a fairly light weight coach to hopefully keep the base truck frame from twisting when driving carefully on ruttted road surfaces. This helps keep the truck's frame and the coach mounted to it in a plane - in that the frame hopefully won't twist front to rear and try to take the plane of the coach floor with it.
As far as lift for ground clearance is concerned, you want to keep overall weight center of gravity as low as possible while at the same time having as much ground clearance as possible. You get this with tall tires .... but with the coach structure and the truck frame at the same time mounted down low as possible on the axles. I see very few 4X4 vehicles with tall tires while still keeping the structure above down low - this takes larger wheel well areas to clear the tall tires but with no - or only a very small amount of - extra spacing increase between the vehicle's coach and the axles. The military Hum-Vee is an example. I once went hunting in a Ford 4X4 PU that was a "low profile" model. It's body height was still tight down close to the axles while at the same time it was a 4X4 with large enough fender wells to allow suspension and steering clearance for taller tires. We use slightly taller tires on our Class C to give us a bit more ground clearance and it doesn't seem to affect acceleration or downhill braking due to it's 4:56 rear differential in an under-weight-loaded chassis. Most Class C motorhome designs have their heavy equipment down nice and low anyway (tanks, generator, storage bins, batteries, etc.) - except that they "look top-heavy" due to their tall coach wall and roof heights. Because of this somewhat low center of gravity, if they have good wide rear tire tracks they're going to remain stable on canted off-highway surfaces whether they're using 2WD or 4WD.
2005 E450 Itasca 24V Class C
