Had the same problem with the thumping. I am connected to a 50 AMP service (which as you may know services two separate 110/120 circuits within the motor home). I never had this issue until very recently. I have a 2016 Tiffin Allegro Red that uses a SurgeGuard 41260 transfer switch. I have two furnaces on the roof. One over the bedroom and one over the living area. The problem occurred when the furnace over the living area turned on. No problem with the bedroom furnace.
I suspected the electrical pedestal my shore line is connected to, so I notified the campground. The fella they sent tested the circuits and found that one of the circuits was normal at 113 volts. The other circuit was running at 133 volts (too high!). He tightened the screw that fastens the aluminum RETURN wire ( which loses thickness over time) and checked the voltage again. This time the voltages read 120 and 125. I haven't had a problem since. Maybe this information will help somebody.
