Always kept my XM antenna on the dashboard of whatever car or truck I had with no problems. A cab over class C might present a real problem for that though. Is the roof of your rig fiberglass? If so, and there is no metal foil sheeting involved, the antenna might be able to "see" thru your roof. Give it a try, just put it in the middle of the bunk and see if you get reception. You might need to attach it to a metal plate to get a ground plane if it's one of those magnetic roof antennas. If that works, find a good spot to mount it internally under the roof. No leaks!
2015 Fleetwood Discovery 40G
2014 CRV w/EZ Tow