I would start by checking the neutral in your 120 volt AC power distribution panel. Disconnect from shore power and have genset off. Remove cover from 120 volt circuit breaker panel. Look in the back for the neutral buss. Look closely at the insulation on the wires. Get some electrical grease, disconnect each neutral one at a time, clean the copper wire making sure it is not dark. Apply liberal amount of electrical grease on conductor and re-terminate. Do each conductor including the main larger one coming from the transfer switch.