You are not going to get the clearances you need, particularly departure angle, with any conventional rear-engine diesel motorhome, because the drive train is relatively long and mounted very low in the chassis to make room above for the house. Because of the length behind the axle needed for the drivetrain, the shortest you will find might be around 32 feet, long enough to get wheelbase close to half of overall length, and put some weight on the front axle for balance.
Off-road motorhomes tend to be built on Class 5 to Class 7 truck chassis, and mostly built above the chassis without basement storage, so that none of the house reduces ground clearance.
Earthroamer XV/LT is about the closest model to the size you've specified, though you'll find much larger (35-45 feet) expedition vehicles built on heavier military truck chassis, from specialty builders like
Unicat, Global Expedition Vehicles. The
Tiger line from ProVan Industries has some smaller options, although their larger models overlap the Earthroamer XV/LT line.
All of these are in the nature of a permanent, rather than slide-in, camper box on an off-road truck.
Class A motorhomes, diesel pushers particularly, are built for the highway and the RV park.