Never had a problem with getting a signal using a doughnut on the edge of the dash.
FYI - a doughnut is a round plastic tray type thing fitted with a collar/ ring filled with sand (or something similar) to create weight around the ring and hold it in place. It has a smooth plastic inner surface to attach the GPS mount. - available at any big box store that carries GPS and almost all truck stops -
Portable GPS Mounts at WallyWorldThere was a recent thread about using a GPS in a Class C and possible interference from the cab over - and the replies were - no problem.
One world of advice on using a GPS - train your DW/Navigator to use the device and functions - doing it yourself while driving at highway speed is not really a good idea. Almost as distracting to your driving as sending text messages.