Sounds like the leg for the 2nd AC which is separate from the first AC might be bad at the transfer switch when it goes to shore power..could be leg on cord or leg at switch itself...check transfer switch connections...especially on the shore power side of it..the bus might be burnt on the shore power side. Or, something with the cord itself...since it runs fine on generator, the generator side of the transfer switch is fine and no cord is used, thus either the shore power cord or the transfer switch itself where the 2nd leg comes off of might be bad. I am assuming this is from any 50 amp power source, even a campground...If this is only from your house, and a campground is fine, it's the power from your house.
