The CB ground plane on my 43' fiberglass motorhome stinks. Has never worked correctly. If I install a piece of sheetmetal around the antenna on the roof would that help the ground plane at all? A flat piece maybe 3' x 3'?? I could screw it down but I think I would have to run a ground wire to a piece of metal near the awnings. Also the piece of sheet metal won't be under the antennae but even with it would that a make a difference?? I was going to drill a hole in the sheetmetal then lay it over and have antenna come through hole. Am I wasting my time??
You can either install a solid ground plane or you can run 6 or more wires, 16 gauge or larger, like spokes away from the antenna. If you make the ground plane radius the same as the height of your antenna or a little longer, that should suffice.
Connect the ground plane in the center to the outer metal conductor of your vertical antenna. There is no need to attempt to connect the ground plane to any other metallic structure or to a real ground rod. It will work best with the one connection in the center, at the antenna.
When you are all done, check the antenna with an SWR meter or an antenna analyzer. Anything less than 2:1 is fine.
Same problem with my coach. Tried a lot of things with no good SWR results until I tried an approach like described above. The first thing discovered is that there is some length of a wire (away from the air conditioner) that produces the best match. Once that length was found, I just taped it down to the fiberglass roof.
Soon it became clear that the signal was stronger in that direction (in my case, forward)and about half as loud in the opposite direction (rear).
I now know that two wires in opposite directions makes the signal about the same in all directions. More wires make no detectable difference. The only trick for tuning is that the wires need to be the same length and bit shorter than one wire to get that perfect SWR.
Buy another RV! No kidding - I have a 23-ft mobile traveler that has aluminum 'shell' that is fantastic for a ground plane (the antenna does better here than in the yard with ground radials!!!). Picked up Australian station at 3AM from Kennesaw, GA one night/morning on my Yaesu 100D and a 75-ft butternut 40/75-meter vertical. The shell is a natural ground plane, no radials needed. The aluminum shell also protects operator inside from radiation....
Ground radials can be made from wire, or from copper tape, or other similar materials. a quarter wavelength would be best, but that's about nine feet on that band, measuring from the base of the antenna to the end of the radials. 18 feet across which I'm sure is not going to fly for you.
That said, adding even less than optimum ground radials will help.
A suggestion: Tune for resonance, match for impedance if needed. The losses even at a 2/1 impedance mismatch are pretty trivial, and contrary to popular belief, 50 ohms isn't magically the impedance of a properly tuned antenna. A quarter wave monopole actually comes in around 37 ohms.
Mobile installs are ALWAYS a compromise, even up at VHF where the vehicle is much larger in terms of wavelength.