If you're only going to be on VHF/UHF then get a half wave antenna. They won't need a ground plane. I use the Diamond NR770HA on all my vehicles. It's a half wave on 2 meters but I'm not sure the effective wavelength on 70 cm, I think it's stacked 5/8 wave.
Those frequencies don't need much of a ground plane. I would think what aluminum and steel is in your RV would be enough. If you start talking about HF (or CB which is also HF) then you get into some long wavelengths that need a lot more ground plane under them.