I've been working on my 1993 Towlite. So far with a brand new battery by bridging the solenoid we popped the camper up. From there I've sanded all the visible connections (except the ones in the cupboards) at this point with just the battery if I flip my breaker switches, nothing happens.

If I plug in my electrical cable to my garage outlet and flip my switches I hear the converted hum loudly and the lights are very dim. After a minute or so I'll hear a click and the lights turn on all the way very bright. After a few moments or so, they will go back to dim or flicker, and a few moments later back to bright.

During this, I took a small desk fan and plugged it into an outlet just to see if it runs, it does. I also accidentally bumped my head on the Air conditioner and it turned on just fine (and scared the living crap out of me)

I sanded the three ground connectors as well underneath.

At this point I have no idea what to do

I am sure some of our retired engineers and electrical experts will be around to help you. I wonder if your converter is bad. They don't last forever.