I recently purchased a 2005 Forest River Wildcat for $1300 that was in excellent condition inside and out. The only issue being the owner thought the slide was shot because the Lippert pump was leaking and he lost interest in fixing it.
I did some research on the ole interweb's and found this was/is a very common problem (of which Forest River completely washed their hands of) with these pumps. A common fix I found was to empty the remainder of the reservoir, pull it off, clean up the mating surfaces, buy a new o-ring, and coat both the reservoir and pump mating surface with generous amounts of Permatex high temp gasket maker, and refill. Haven't had an issue since.