If the motorhome is sagging in the rear from too light spring rate, add a leaf or 2 to the spring packs.
If it has plenty of rear spring rate and simply needs more ride height, add spacer blocks under the spring packs.
Most factory stock 4wd trucks use the same rear springs as their 2wd brothers, but use 2"-4" spacer blocks under the rear springs to gain the additional suspension height 4wd's are known for. The blocks are tapered and installed in one direction only, which rotates the front of the axle up slightly, to line up the driveshaft. You can do the same for your motorhome as is done with 4wd trucks.