Mark,
A 50 amp Surge Guard surge protector will not connect your coach if there is anything about the shore power source that is out of its specification guidelines. In the case of Jerry's coach, he had power on the incoming neutral leg, and that is out of specifications for the Surge Guard. Therefore a Surge Guard would not have allowed the coach to connect to the power source, and no damage would have occurred.
I have a Surge Guard 34560 hardwired into my coach, so all power coming into the coach from a shore power source has to go through it. It does not matter whether it is from a 30 amp plug or a 50 amp plug, all power has to be within specifications or it will not connect. If the shore power source has excessively hi or low voltage, power on the neutral leg, or power on the ground leg, it will not complete the connection.
Gerald