I did have a "somewhat" strange event a few weeks ago when camped at Brown County State Park (Indiana). The extended week-end we were there it rained, and rained, and rained. When we broke camp to come home, I was able to begin packing up, but everything was wet. I did my best to clean off my slides, but they were still wet. I did get the leaves and sticks off, but there was still water, and even the squeegie couldn't keep up with the rain. I had no choice but to pull the slide in and let the rubber seal squeegie the water off. No problem. Came home all was well.
Once home, I set the trailer up in the drive way, like always, pulled the slides out, plugged in the electric, flipped on the air, set up the television and continued enjoying the camper. (been sleeping in every night this year so far).
Anyway, next day we're sitting there watching television and I look up at the ceiling. It's all spotted like a bazillion bugs crawled over it. I took a wet cloth and started wiping the dirt off. It was dirt, not bug goo.
I then noticed it ran ONLY the length of slide out. Then I look at the ceiling above the space where the other 2 slide-outs sit when retracted. And sure enough the ceiling was spotted with dirt too. It all washed off easy enough, and the ceiling is all nice and white again.
Sitting there watching television I began to question what caused that dirt on the ceiling in the first place. Then it hit me. It had to have come from the wet roof on the slide out and the loose dirt. Once we took of and hit the road from the campsite, the bouncing of the camper must have splashed the wet and dirt from the slide to the ceiling of the camper. It never happened before, and hasn't happened since. But this was the only time I've ever retracted slides while it was still raining.
So, if there is another reason for cleaning the slide roof before retracting, I suppose this is one. It will keep the dirt from splashing on the inside of your camper if it's wet!