I installed a pair of 110 watt Siemens panels on my Suncruiser along with a 50 amp charge controller. I now have them, plus two more for a total of 440 watts, on my Bus. On both of these RVs it was quite easy to mount them to the fiberglass roofs. I did not use screws, however. While the Tiffin roof is fairly thick, the Winnebago roof isn't. Instead I used 3/16" diameter stainless steel rivets. They know over on the backside and you won't have to worry about any screw threads stripping out of the fiberglass. Lay a bead of self-levelling caulk down under the mounting brackets and be sure to caulk up the rivet heads when you're done. They held up very well on the Suncruiser and should do the same on the Bus. You can thump the roof and see where the crossmembers are. They are pretty far and few between so you shouldn't have a problem avoiding them.
In order to run 440 watts of solar power you'll be running around 45 amps of current, depending upon the voltage - which is determined by your battery condition and how bright the sun is. Two panels is half of that. If you are running two panels you can get by with a 30 amp charge controller. This would require #10 wire to get to the batteries. If you think you might expand them later on run #8 wire right away and buy the 50 amp charge controller. That's what I did and when it came time to transfer them to the Bus (and add two more) all I had to buy was two more panels. Just run the wire down the refrigerator vent and then down to your charge controller and then your batteries. be sure to lay a rug over the panels when working on the wiring or else you'll find out how to make sparks when working on the wiring.