I have found that by slightly elevating the front of the TT maybe a bubble or two before putting the slides in, will also help in the leftover "pools" of water to drain that settles on the low spots after rain. Then you can re-level off the TT and slide the slide(s) in and as the others have mentioned, towel off the moisture leftover. I use a chamois to soak up the drips on the crevices of the sides and the top just to be extra careful. ( I'm paranoid
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I was told as long as I do this practice of retracting and putting the slides out while mostly level, I would not over work the motors (electric) and not put the slides out of alignment. Just make sure to re-level as close a you can, before moving the slide(s).
With what they've said and I added, I have noticed when the slides are only partially out, the seals are not completely settled everytime, and could cause leaking into the TT, especially at the corners, as Festus2 said, my slide roofs angle slightly away once fully out to shed the water, not angled at all when only partial. I wouldn't trust it as a regular practice.....