There is a common misconception with respect to RF and/or DC grounding, and its relationship (or lack of it) to a ground plane.
Just because you run a ground strap from the antenna mount (the ladder in this case), to a hard point on the vehicle, doesn't automatically make it a ground plane. In fact it doesn't.
The best way to think about vehicle ground planes is simply this; it is the metal mass under that antenna that counts, not what's along side. The second part is, the more metal mass the better.
Yes, you can make contacts with the lossiest of antennas, as any Outbacker or ATAS owner can attest to. However, if you want consistent communications, then you need a decent amount of metal mass under the antenna, and a ladder at the rear of an RV isn't it.
Alan, KØBG
www.k0bg.com