I have a Coachman Catalina and the same situation arose with my slide, I did not notice it when I picked it up. The rear gasket was completely compressed, the front barely contact the body of the trailer. I am not sure what made the slide "jump the gear" or if it was like that when I purchased it. I did get a branch stuck in the slide when I was closing the slide a couple of trips back, not sure??????
I unbolted the slide gear on the front slide bar, it sounds easier then it was, measured the back position of the slide when it was open, set the front measurement to the same by pushing the front of the slide in to match the rear measurement, now both ends of the slide meet the trailer wall at the same time , and compress the gasket the same.
took me about 2 hours to align the slide and button everything up. Why did I do it myself, well the dealer I purchased the trailer from is 5 hours away from me.
Good luck with yours, and many years of happy and safe traveling!