Bob,
I had a similar problem on my Onan 7500. In my case, the neutral terminal bar on the generator connection wiring block had broken from stress. This wiring block is on the corner of your generator. it connects the generator output wiring to the coach wiring. It is a black plastic block with fins separating the individual wires. The metal terminal bar goes between and connects the input and output wiring for each individual wire. Even if you are not comfortable with electrical wiring, you could visually inspect for this problem. The neutral wire is the white wire on the terminal block. I suspect Onan had a problem with these terminal blocks because the replacement for mine was more robust in its construction. Do this inspection with the generator off for safety.
The above procedure is a little more complicated so I would first verify the transfer switch is not the problem. If you can get someone with electrical experience to help you, while you have the transfer switch open and the generator running, you can check the generator side of your transfer switch for a good neutral. If the neutral is good there, then check the output of the transfer switch. It you don't have good voltage there, the problem is your transfer switch. If you have good voltage through your transfer switch, the next step is to check your main AC distribution panel inside your coach. Given what you said about your electrical experience, I would recommend some professional help if it gets to this point.
Good luck,
Bill
