I'm not sure how your MH is wired, but you may want to check the 20 amp outlet your MH is plugged into. It may not be grounded. You can use one of those plug-in circuit testers with the trouble lights built in. They are sold at Home Depot, Lowe's or other hardware stores. If you get a clean bill of health at the outlet, move the tester to the other end of the extension cord and plug it in. If it's good to that point, plug the MH into the extension cord and plug the tester into a GFI outlet in the MH. This process should tell you where to start.
2004 Tiffin Allegro 27.5 ft. P32 18,000 lb GVW. 8.1 liter. Workhorse chassis built May 2002. 21,000 miles. 2012 Jeep Liberty Toad. RVi2 brake unit.