Are your slides open or closed while in storage? If open, I'd close them. If closed, open them. Either way, treat the rubber seals with a rubber seal treatment, or use baby powder to soften the seals up a bit, and close them up again. Do the inside seals and the outside seals.
More than likely, they just need to be exercised a little. If rubber does not move, it gets hard. Once it get's hard, it doesn't seal good any more. Simply moving the slides will help make them flexible again But if the slide is open, definitely close them.
2019 Montana High Country 375FL
2014 Chevy Duramax HD 6.6 - 3500 Diesel Dully Long bed Crew Cab