Easiest place to put it is near the top of and inside one of the cabinets above or beside the driver. The fiberglass roof is essentially transparent to the satellite signal. You should really mount the magnetic antenna on a 6" square of metal - the metal serves as a ground plane and will improve your signal strength. You can then run the antenna cable down one if the pillars to the radio. I would also mount it about 6 inches below the roof to avoid putting directly under one of the steel members supporting the roof.
09 Journey 39Z
I duck taped a piece of metal on outside mirror arm and the antenna magnit stuck to it and then ran the cord thru sliding window. my sound sloppy, but black duck tape hides everything. magnet wouldn't stick to any surface on the outside of motorhome. I may look for a better mounting system, but this is working. Dave
Tiffin 32CA 2012
2007 Jeep Liberty
If you want reliable reception, install a Sirius/XM marine antenna. I have tried using the supplied antenna with and without a ground plane at the top of the cabinets, always had dead spots while driving in certain directions.