RV Cover Rolls rotates around RV during high winds through the winter

Hopefully someone else has this problem. I have a 36' Class A, living in Pacific Northwest. In the late fall I winterize and cover the RV with an ADCO style cover, with straps that go under the RV. The cover fits well and the straps are snug but not tight. 

Any sort of heavy winds and the cover starts to rotate clockwise around the RV. Over the course of months it gets worse and worse, until what should be the bottom touching the ground is mid-way up the rv, making it impossible to enter the RV without first releasing all the straps, climbing on top of the RV, and resetting the tarp. Its a very unsafe feeling, especially in the winter.

I've tried tying it off to a fence but it just shifted and rolled the other way. Recently I tied one of the straps to a very large cement block, but instead the strap snapped off. Its really not that windy here very much. Am guessing I'm missing something and others have solved this problem?

Cheers