if you support it on blocks, support the suspension not the tires and not the frame. Tires will get flat spotted (may recover) but a hanging suspension will not recover. Pack it with mothballs so rodents stay away. Id fill the tank to full (add stabil for conscious sake) and leave the oils in place. A full tank wont rust under the fuel line. take the tires off and put them in a sealed bag in your shed. ozone is the tire killer. 5 years is not bad. Ive started much older sleepers with just a squirt of gas and a jump. If you could start it quarterly that would help but just to stir the fuel rails. If your going to cover it, make a framework that will keep tarp off it. Ive never had a tarp last more than 2 years out in the sun. If your really concerned about the moisture, do you live near a dock? They could wrap your truck in shrinkwrap like they do the snowbirds yachts. here is a 17x31 foot one piece wrap, 58 bucks! totally waterproof and video.. 17′ X 31′ 6MIL (DS-176031) Blue | Shrink Wrap | Dr Shrink
