^What Lynmor said.
In particular #3. Plugged in continuously charging. Does your converter also function as a battery maintainer?
If it does not, it is overcharging the batteries due to little to no draw on them and full charging amperage.
1 of 2 Solutions there.
1. Disconnect batteries when not in use and periodically charge them or put a maintainer in them.
2. Depending on your converter, you can add a Charge Wizard to progressive dynamics converters. Possibly similar for other brands that monitor voltage and maintain the batteries.
Corrosion is leakage. May not come back if not overcharging. Or may.
In the past I’ve cleaned up old batteries around the terminal base and put a coat of rtv sealant around them. Stops minor leakage for a while.
