When we bought our GT, the owner had left the slides out all the time it wasn't actually on the road. Where he parked it, the living room/dinette wall faced south. When we did our pre-purchase walk-round, I noticed the front edge of the slide topper was frayed.
We later found out that the whole topper had rotted out, probably from solar exposure. There was only the netting substrate left. As a result, we got a major rain-water incursion. Fortunately it only afftected the overhead cabinets in the slide-out, thanks to a large package of table napkins that soaked most of it up.
We did get mold in the cabinets as the weather warmed up and I ended up removing them and building new cabinets. As a result, we keep the slides retracted unless we're on the road or someone is staying at our house and spending overnight hours in the RV.
Frank and Eileen
No longer RVers or FR owners