My guess is that your SurgeGuard has bitten the big one. It sounds like it is not allowing one leg of the 50 amp service to get to the trailer.
This happened to me last summer. The main A/C would not work but the small A/C in the bedroom would work. Upon further investigation we discover several other appliances/outlets that wouldn't work. It seems that when my SurgeGuard died due to water intrusion last summer it also damaged the 90° dogbone attached to the main power line. When I hooked the power cord up without the dogbone, everything worked like it should.
I would checked both legs coming out of the SurgeGuard for power. I'll bet one of them isn't working. And then I would pitch it and buy the PI one.