We have a full wall slide, (same family of coaches) and no problems. The water collecting on the slide topper is a problem with the extended length.
I made a pipe hook to put on top of the topper to pull the end of the topper down to help drain the water. When you park the coach, note which end is higher and keep that end high while leveling. That will help the rain to drain off before it has a chance to pool-up and start a sag in the topper.
Also check the topper for water and if there is water pooled up, dump the levelers and either raise the front or rear jacks (depending on pitch of coach without jacks) to run the water off. Then retract the jacks and retract the slide. If you have time, let the toppers dry before retracting slides.
2019 Fleetwood Discovery LXE 40M w/2012 CRV toad