IF it does fine on Propane than the suspect is indeed the electric element.. That can be a 'Witch' To pull out (or not depending on the install but it is a fairly easy repair. Technically.. Just put in the same wattage
Other thigns that may affect
Any obstruction in the path from the outside lower panel to the upper panel (roof or top) on my Rig I'd put fans in outside and they blew crud up into the heat exchanger. When the unit failed (Different cause) there was a bit of fiber on the bottom of the heat exchanger.. Compressed air would have blown it back down no problem but we deleted the fan.
I did add a set of 3 fans inside (dometic) from RVCOOLINGUNIT warehouse dot com
These are designed to attach to the fins INSIDE the RV.. Mine "Eat" off the interior light circuit not fed out the drain tube since I have a Dometic I did not want to do it that way.
They help keep it "Defrosted" and so far are doing just fine.. I may move em to a different fin but they took 'er down like 5 degrees as they blew the frost off. (Well what was left of it)
Home was where I park it. but alas the.
2005 Damon Intruder 377 Alas declared a total loss
after a semi "nicked" it. Still have the radios
Kenwood TS-2000, ICOM ID-5100, ID-51A+2, ID-880 REF030C most times