Onan Marquis 5500 put out too much voltage

Might be comparing apples to oranges, but my Generac Guardian 5500 recently started acting up and was putting out 153 volts after driving through much rain. Removed cover to expose control board and found it was covered in a thick layer of dust and grime and was damp from the previous days rain. After cleaning the dust off, I sprayed the contacts and plug with electronic contact spray. That fixed the problem.

You said you replaced the control board, but did you clean the plug?

Also, after cleaning the control board, I discovered a tiny potentiometer which, according to the manual, is used to adjust the output voltage.