Back in the days when motorhomes had 'west coast' trucker style mirrors, mounted with stainless or chrome steel tube, the CB antenna(s) mounted to the mirror brackets. Sadly, with the advent of fancy all plastic 'aero' mirrors, that is no longer possible...
If your motorhome has a roof rack or a ladder, you can use a mirror style mount bolted to the rack or ladder. It will make for a very long coax cable, but so be it. Should work about as well as an old style mirror mounted antenna, as far as ground plane is concerned.
If you're not towing anything, or have an extra receiver box hitch available on the bumper, you could make an L-shaped pole that slides into the receiver hitch tube and extends up to roof level. Either a 4 sq ft piece of thick sheet metal, or 3 or 4 horizontal radials about 2 ft long each will act as a ground plane on top of the pole, with the antenna mounted above. An interesting but effective home made antenna that will do the trick for short ranges.