Leaky !@#$% Roof Air Conditioner

1987 Fleetwood Bounder. Previous owner must've had trouble with it too because there is slight ceiling damage from moisture, but it's minimal. He replaced the gasket before I bought it in 2007. I replaced it when it leaked in 2008, again in 2012, and as soon as the Oregon rain began this fall, the !@#$% is leaking like sieve again.

I can't believe replacing this gasket every year is normal. What's the catch?