Inverter is pure sine, and it is the Inverter plug that trips, not the trailer plugs. Works fine with generator plugged to trailer.
Will try BFL suggestion and unhook the inverter ground. I ran the previous inverter on 30 amp trailer without a ground.
Back to trailer on the weekend, no internet there, so Tuesday before I can confirm.
Just talked to electrician who will hardwire the inverter for me, and he thinks it might have something to do with the phasing on 50 amp service. Although works with generator. Any thoughts on that?