I would certainly NEVER say that water intrusion cannot happen. But we have camped in and driven through very heavy rain (so with slides both in and out) and have never had a drop get in our camper. So I don't think I would let that chance stop me. Keep the roof in good shape, and regularly inspect the slide seals and you should be in good shape. Good luck and enjoy the RV lifestyle.
Skip & Susan Orrell
2013 34 RLSA w/ most of the options
2013 Chevy 3500HD, D/A, DRW, 4X4