We have had the same problem. We spoke with Fleetwood service while in Decatur having other items taken care of and their solution was to raise the front of the motorhome to drain the water that collected on top of the slide (under the topper). We were there multiple nights and it just so happened it rained one of night after I had mentioned the problem to them. When the tech came out to get the motorhome, he got the shower! And it wasn't a gallon. We're talking 5 gallons!!! After cleaning up the mess, we proceeded into the shop for the additional repairs.
Unfortunately, we have not actually "seen" them do the raise and drain, but we have had opportunity to try it ourselves. We raise the front of the motorhome using the jacks' manual mode. We then retract the jacks, re-level, and then pull the slides in. Once in, I take off my shoes, and while standing on the driver's seat I use a flashlight to confirm there isn't any water left on top of the slide. If I see a lot, we repeat the procedure. If there's just a little water left, I have a large beach towel I roll up and put at the end of the slide on top behind the driver's seat to catch any leftover water.
Note that sometimes you have to bring the slides in a little at a time to drain the water that collects on top of the slide toppers.