What bands do you plan to work out of the RV
For practical reasons vertical antennas are not going to be roof mounted for HF, only VHF and then likely not for six less it folds down
For 70cm you only need a 35cm circle for a ground plane, tha't is a bit over 1 foot diameter.
For 2 Meters a 1 meter circle (38" in this case, or 19" radius)
Just divide the wavelength by 2 to get the circle size for the ground plane.
For HF. go with something mounted to teh bumper/frame, if it's a TT mount to the tong, if it's a 5er extend a frame rail enough to attach something like a Screwdriver.. OR do what I did (not the best option) and use a Auto-Tuner (marine grade) and attach a long wire and throw it up in trees (not always allowed).
For 10 or 20 meters you can string a dipole along the roof.. I do plan on doing that some day, I currently have a side rail on one side of the roof (1/2 inch copper tubing, soldered (electrical solder) connecitons, hooked to the Antenna Tuner with stranded copper 10ga (what I had in stock)). and plan on putting a second rail (PVC) also half inch painted on the other side, it will hold a 10/20 meter dipole.
But for most hf bands. roof mounts just are not the best choice.
Though a DDRR antenna might work for 10 or even 20 if properly designed.
NOTE that contrary to popular belief, Dipoles DO NOT NEED A GROUND PLANE. There are also VHF antenna designs (and UHF) that do not require a ground plane.