It's not necessary to unload, but it would be a good idea to remove anything that you don't want to grow legs while in the shop. It's never a bad idea to do a walk through, inside and out, with a camera to document the current condition and personal property.
While not in use.. absolutely slides in. Slide gaskets are not designed as an all weather airtight seal. It also puts undue stress on your rigs structure and slide supports. Even while camping, if there is severe inclemental weather, many will pull their slides in whenever possible.