Go to a NAPA store, and buy a spray can of automotive clearcoat. Make sure there is no grease on the aluminum. Clean it with either paint thinner or lacquer thinner and make sure it's dry.
Two coats should work just fine. I also use this to touch up the exterior from time to time.
Old age is coming at a really bad time!
1996 34' Excella 1000, interior totally redone, 2003 Dodge/Cummins HO, Diablo tuner, 115 gallon aux fuel, Bedslide, Airsafe/Reese Dual Cam, and a bunch of other stuff