Just my opinion but having the inverter/charger diagnosed and repaired could be very expensive. After you find someone who could fix it, compare that price to possible prices from the multitude of salvage store around the country.
I have a list of Surplus/Salvage yards in my files but couldn't find it after a quick search. I am in the process of converting all my reference stuff for RV's to PDFs and right now it's a mess.
If part of you reason for wanting to retain you vintage inverter is the possible need to move to a new location, and mess up the looks of stuff. There is a guy who is often mentioned here who know what to buy and how to install the new guts of most manufacturers inverter/chargers without any mods to the house.
Google converters and you'll come up with his name.
You could try warming your inverter in the oven. Use a low temp, actually very low temp for s couple of hours. Take several continuity readings before and after baking. If no changes occurs, you're probably toast.
"I travel not to go anywhere, but to go. I travel for travel's sake. The great affair is to go". R. L. Stevenson
David Bishop
2002 Winnebago Adventurer 32V
2009 GMC Canyon
Roadmaster 5000
BrakeBuddy Classic II