Best storage is inside storage. Next best, covered storage. Open to elements is better than a tarp. Your cover is good, and I wish I had one. I've not heard of anyone in my area getting more than one year out of one though.
Don't listen to what others say about not supporting the bottom of your camper. Your manual is the same one mine has. Floor must be supported at a minimum of 3 points by a beam that is a minimum of 5" wide and supports across the entire bottom. They even diagram a stack of 2x6's fastened together into a 8x6. Look at your owners manual, or find one online specific for your camper.
Beware of the plywood X, it's a fantastic idea, but not really going to work for any real weight. Plywood is not designed for any shear strength in that configuration, and is difficult to seal properly from the elements making it even weaker.
I've had good success with landscape timbers (4x6), cut to 52" lengths supported by cinder blocks for height, and shimmed to level by boards/shims/leveling plates,etc.
Silver's covers are a fantastic idea, I spent the winter gently sweeping the snow off the roof of my camper every time it got over 12".
I'm really hoping for inside storage by next winter, and ideally would prefer at least covered storage year round. I don't want to have to replace UV worn plastic and rubber roof any time soon.