Absent any better ideas, I took the whip antenna I once used on my
Honda Pilot and mounted it on a 2-by-2 foot piece of sheet metal from
Ace Hardware. I attached the latter to the RV top using industrial 3M
dual lock tape at the corners, and strips of 3/16" weather seal
underneath to prevent rubbing.

I have no clue how big the ground plane is supposed to be, or if it's
needed for transmit as well as receive, but I could hear pattern
traffic at an airport 30 miles away.

2NO