Just to verify things: 1. You have a 50 amp wired trailer? 2. You are connected to a 50 amp shorepower outlet? If connected to a 30 amp shorepower outlet, have you checked (swapped) your 50 to 30 amp adapter in the power feed line?
If indeed you have a 50 amp wired trailer connected to a 50 amp shorepower cord, I would take the metal surround off of your inside breaker panel, and do some voltage measurements with your voltage tester. Nearest the double master breaker you will see the shorepower cable with 4 large wires coming out of it. The green wire is ground and should be connected to the ground buss, the white wire is neutral, and should be connected to the common neutral return buss, the red and black wires are the 2 hot 120 volt feeds which should be connected to the double mains circuit breaker. Check the voltage where these wires connect to the mains. There should be 120 VAC to neutral/ground at both of these connection points. This double mains breaker can have problems in connecting to the 2 buss rails in the breaker box causing 1/2 of your power to be missing. On my breaker box every other branch breaker is on a common phase feed, so checking the voltage out of the branch breakers at the wire feed connection points is a good way to make sure that both phases are getting through the mains breakers to the buss rails and the branch breakers.
Making these measurements should give you a better idea of what is going wrong, at least upstream (before) or downstream (after) the breaker panel. Be safe when making these measurements, and shut off the power while removing/replacing the breaker box faceplate.