I looked up the antenna/booster. I think your best option for a Campster
or most any fiberglass trailer will be a pole mounted to either the rear bumper or behind the hitch in front. I'm not sure whether this antenna requires a ground plane, but the frame of the trailer can serve that purpose. They mention attaching it to a rear ladder. A pole should work as a substitute.
Thats the approach I've taken with my ham radio antennas.
If you really wanted to use a mag mount, set up the booster in your tow vehicle and set the inside antenna where you can pick it up from inside the trailer. You would have to figure out how to power it.