As stated, it is most likely that the camper is obstructing the signal path.
Since low profile antennas for Sirius are relatively inexpensive ($20-$30) it may be possible for you to purchase one and install it on top of the camper. Run the line down the side and up through one of the drain holes in the truck. Just cut an "x" or "+" in the drain hole to push the wire up through. When you are traveling with the camper you would use the antenna attached to it. When you are not traveling you would reconnect the truck Sirius antenna.
Just a thought. Good luck.
Wayne MSGT USMC (Ret) & Earlene (CinCHouse)
2008 Winnebago Destination 39W
It is what it is, and then it is what you make of it.