Steve gave you ideal advise. However, let me emphasize a couple of points.
First, never use existing vehicle wiring to power any amateur radio equipment. If for no other reason than to minimize RFI problem, ingress and egress.
Mounting a 2 meter antenna "CB" style on a mirror mount is a prescription for poor performance and possible RFI problems with respect to inadequate groundplane issues.
If height is a problem, you'll find that an ordinary 1/4 whip mounted atop your motor home (if it is aluminum) will perform beyond expectations. If it is fiberglass, I suggest you use an antenna designed for that purpose. www.radialllarsen.com
is a good place to start.