I can't say either way on the ground plane thing. I've seen arguments that it worked when a piece of metal was added and other arguments that it isn't needed, just like these posts. I haven't tried the metal on the dash but may do it just to see. The biggest issue I know of with putting the antenna on the dash, windshield or anywhere other than the roof is the satellite location. The Sirius XM satellites, like most others, are orbiting over the equator. Since I'm north of the equator, with the antenna sitting on the dash, I get great reception when driving south, pretty reliable with heading east of west and absolutely no reception when heading north. Fiberglass in the RV skin is transparent to the signal, but mass will block it just the same, so upper cabinets, structural framing, etc. will kill the signal. Wish I knew an easy way to get it on the roof but I don't want to go drilling holes.
2015 Legacy SR340 360RB
2014 Jeep Wrangler Unlimited Toad
Gigi, Poppy and Sadie Lady...On the road, but not full time!