My roof just above the driver is steel, so the magnet works great. I just ran the wire down the outside of the roof, and in through a hole I drilled in the rubber around the drivers side window. I sealed it up w/ clear silicone. I also put some clear silicone under the magnet, for some extra stick(belt and suspenders approach)!! You might try a painted steel plate riveted and glued to the fiberglass, and then silicone like I did. I would definitely put it on the top of the roof though. On mine, I have it wired so that as soon as I turn on the battery switch, the speedometer is powered, so that I don't have to wait to acquire the satellites. Now that the air bags don't deflate(at least the back ones), I don't have to wait as long to build air pressure, before I can go.
Unfortunately, I still don't have the bus inspected, as they said that my steering ram is leaking, and it has slop in it, so it still fails inspection.
When my wallet recovers from their bill for fixing the brakes, installing 2 air bags& leveling bags, and a couple of wheel lugs---I will get the parts to fix the steering and do it myself. They did find that the one of the rear slack adjusters was all the way backed off! The brakes really stop now!
I want my rig to be safe even if the interior build out has to slow down to do it.