No adjustment that I know off for just the center or middle of the slide. If you are concerned enough or are having leaks, you may be able to shim the seal to close the gap at the bow. May be time consuming and take a lot of patience. Basically build a bow into the slide to match the coach. Moving the slide out and using a string line you could detect if the bow is in the slide or the coach wall. Probably doesn't matter which it is, the easiest solution is to shim the slide seal.
1998 Min Winnie, 2000 Winbago Journey, 2015 ACE 29.3
2016 Thor Miramar 34.2