Turn off the main circuit breakers, on my 2551 (2015 version) there are 2 "main" breakers. One for shore power and the other for inverter outlets. With them off, see what the inverter does. If you still get an E5 error, I would suspect the inverter itself. If no error, turn the breaker for the inverter outlets. If you get the E5 error, something is overloading. I'd unplug the stuff in the passenger side front cabinet (DVD and TV and surround sound) see what happens. If all is well, turn on the other main breaker.
Personally, I would suspect the inverter itself. I had a similar problem a month or so after I got my PC and when I took it in to Phoenix, Bob tested the inverter and it was bad and he replaced it.