One of the best no-ground plane CB antenna is from FireFly featuring molded side mount. When you install the 4 foot top tune-able antenna install a SS spring first.
Be careful not to mount your antenna too high
The following is my invoice from truckers-store.com
http://www.truckers-store.com/store-..._40491594.html
lg3m2-lg4m2 FireFly ® No-Ground Plane CB Antenna Kit Featuring Molded Side Mount 46.95 1 46.95
Options: Antenna Color: Black
Length: 4 foot length lg4m2
aud306-auss3h Accessories Unlimited Antenna Springs - Four to Choose From 10.95 1 10.95
Options: Choose Your Spring:: auss3h - Heavy Duty Spring
Subtotal: 57.90
Shipping: 18.72
Tax: 0.00
Grand Total: 76.62
Good luck...Remember that on an RV you don't have a good ground plan like the truckers have. No-Ground Plane CB Antenna is the only way to go