Have you replaced slideout seal/gasket yourself?

It depends which gasket you are having problems with and how it is attached.

I replaced the large "wiper seal" that spanned the gap between the slide and MH on my last rig. The cost of a length of seal to do the job was about $225. In this case, it simply pressed on a flange that surrounded the slide. It was quite easily done from the inside with the slide partially extended, though care was required to get the corners right.

Don't know if this matches your circumstances or not.