If you choose to mount your CB antenna on a portion of the roof or the fiberglass body of the RV, you must use a
No Ground Plane antenna. The "Firestik" brand which Nehog mentions is good choice. "Firefly" is another good brand. They are available in 2-foot and 3-foot lengths. The longer antenna will perform slightly better.
When a CB antenna is mounted to the metal body of a vehicle, the antenna uses the metal of the vehicle as a "ground plane" which basically forms half of the signal radiation system. If you mount the antenna to fiberglass, which is not reflective and does not conduct electricity, that ground plane is missing. Performance will be
terrible if you use a regular CB antenna.