I would store them closed, I also have a Truck Camper, that I forgot to close the slide and it has a topper also, the snow pushed the topper down on top of the slide which then showed water marks on the slide.
Rich and Angela, Lab Dogs, Ruben, Ace
2015 Voltage V3895
2006 Lance Truck Camper 1181
2019 GMC 3500HD 4X4 CC DRW LB Bags Husky 26K Hitch Can Am Commander 1000 ATV
It supposedly doesn't really make any difference. I had a Cardinal LE that I sold and it moved to AZ. It spent 3 years out there with the slides out, then they were pulled in and it went somewhere up around Denver. Most people that store at a storage lot aren't going to bother putting the slides out when dropping it off and then they have to go pick it up. At my house there is covered storage with hookups so I would leave them out... if I had them. In your case it might be better to leave them in.
__________________ There is madness to my methods
2015 Coleman CM16FBS(traded) 2016 Concord 300DS
2015 Ford Fusion Hybrid following along
i vote IN sealed either way, but, the sealing surface is on the "outside" of the side wall, with the slides "IN"... and keeps the UV , wind, rain, off the seals. last, the slide roof isn't there in the first place if your trailer becomes out of level