Need advice for groundcrew vehicle antenna for 123.5

I have a Roadtrek (small Chevy van converted to a mini-motorhome) which has
a fiberglass upper roof.

Wanting to set up good ground to air communications ability on 123.5 and
123.3 with a Tait mobile radio in the vehicle. The problem is establishing
a ground plane with the fiberglass roof.

Other than a magnetic mount antenna on the metal hood....any suggestions?

Wouldn't mind a fiberglass whip type set-up if the performance gains
justified it, but I would like it to be functional while driving....also
don't know how to establish the ground plane.

Appreciate any advice. Thanks!

J4