2002 Newmar Kountry Star reoccurring electrical problem

Monitor voltage and check for lose connections especially on the neutral. Look for heat related damage such as discolored insulation in the main breaker panel and the transfer switch.

I would test with a resistance load on each side of the panel such as space heater or hair dryer. Monitor voltage at the panel (both sides) as you adjust the heat levels on each side.

Bit of a methodical process to divide, eliminate, and isolate this sort of issue. You may need an electrician.