What ceiling material do you have currently? My motorhome has a sort of low-pile carpeting, but I'm guessing you have something else as I can't imagine trying to paint what I have.
I rather doubt ordinary household mud and paint would hold up well to RV usage; there's too much shifting and bending and flexing.
I agree with DutchmenSport's suggestions assuming you have some sort of wallpaper-like material It would probably not hurt to scuff the surface well, perhaps even sand it lightly, and definitely use some tenacious paint that sticks to everything. Do make sure whatever sort of texturing agent you use can take some movement without crumbling off.
Another option might be finding some sort of fabric or other material that has the sort of look you want and attaching it with e.g. contact cement. Naturally, lighter weight materials are preferable to heavy ones.