Going to try to explain this....LOL...Need help

How about also cleaning up any corrosion where the battery neg post is grounded to the frame plus jumpering the "fuse?"

with shore power on, what is the voltage at the pos wire that was on the "converter side" of the "fuse" and a clean frame ground (tongue)?

Corrosion may have gone up under the insulation at the ends of those wires that were on the corroded "fuse" so you might need new terminals farther up where the wires are clean.