This is what I had to do when I had a truck camper.
Got a antenna with a magnetic mount and installed on the edge of the hood (you can see it on the hood in this picture0. This was so long ago that the XM unit was a add on that played through a cassette device. It worked very well even in Canada in 2005. I do think, however, you can unplug the roof mounted antenna and plug in this type of antenna on vehicles which have satelite built in to the radio.
On my newer Class C, Nexus installed the antenna in the grab depression to open the hood. This works really well.
Chris
2018 Nexus Ghost 36DS
360 Cummins, 3000 Allison
2016 Ford Expedition