FWIW, switches were common in the '70's on PD converters, then there was an auto relay option,
The reference to a huge capacitor makes me wonder if you have a ferrorresonant converter, which used big honkin' capacitors as filters/battery eliminators.
Picture of the breaker and fuse panel would help, along with any brand and model numbers.
But- I would look for a fuse or breaker in the battery leads, and double check the battery grounds.
("Older RV" means different thing to different folks- I have people tell me they have an older RV- what year?- 2005.....not my idea of old :) )