Tire covers are mostly ... For looks not protection.
As someone said Dry Rot of the tires is the major concern. You might want the bearings repacked.
Insect infestation is another issue you might want to bomb it oh say a month before opening up and re-stocking in in the spring... IF it's outdoors take care of roof and siding (Wash roof and wash/wax sides as you normally would)
Make sure batteries are maintained either plug in 3-stage converter) battery minder if not a 3-stage or if you can't plug entire rig in) or battery minder with batteries on a slab of scrap wood in garage (Why set them on scrap wood? Well there is a myth I won't bore you with but if the battery leaks, even a tiny amount. Scrap wood is cheap. Floors and bench tops are not. at least relatively)
Oh my Apartment's 12 volt system consists of 3 big batteries with 3 stage charger. they are on a slab of plywood in a plastic tub.. Full protection. (I take my own advice)