I don't do anything special, unload anything that will go bad if it gets frozen, clean it up, top off the water in the batteries, check the roof for cracks in the sealant (fix as necessary) and plug it in.
some storage places have 15amp power available, that is one thing to ask about, another is how easy is the access. can you go any time with a pass code or do you have to go during working hours.
if you cannot plug it in and it gets real cold, then I recommend taking your batteries home in a heated garage or something on a trickle charger. if it doesn't get to cold, just pull the main positive and negative wires off them, but you have to be prepared to go check them every month and charge them up as necessary.
covers are hit and miss. they are large and bulky so you have to have a place to store it when you are using the camper and they are not easy for one person to put on and take off. the other issue is if you get a cheep one it can put wear spots on the rv, and trap moisture. a good one shouldn't but your going to pay for it. if you do a good clean and wax with a UV protectant wax every couple years you should be fine that way. tires, you can get tire covers, they are cheep and depending where you live they might be worth it or they might just make you feel better. for me they would just be a feel better thing as it gets very hot here in the summer, and kind of cold in the winter, but the UV must not be as strong as Arizona or Florida because I can easily go 10 years with out tire issues related to the sun.