If you can find a piece of copper window screen, maybe at an older hardware store that still stocks same, it can be applied to the underside of a fiberglass camper shell. I used 3M adhesive plus a few strategically placed pop rivets with washers on the screen side and a bit of sealant on the outside and it worked very well. Bond the screen to vehicle ground, of course.
Friend of mine removed his campertop and painted the underside with conductive paint -- can be found at www.stewmac.com
as paint used for shielding guitars, that's where he got his, likely other sources. Then he shot a second coat of auto paint the same color as the original inside of the camper shell to protect the flat black conductive paint from abrasion and make the shell look good in there. Left a small area of the conductive paint in the clear for attaching a grounding braid over to the truck frame.
Personally I like the copper window screening better, especially if you intend to run QRO mobile.
Vertical antenna needs that ground plane right underneath it, these methods can do that for you.
Likely your VHF performance will get a lot better, too, especially for longer distances.