We had the same problems here in the Southeast with our RV cover. Best available at the time.
I see in ads new cover with panels to let air through instead o billowing in the wind. Every time we had high winds I had to retighten the straps underneath but it was never enough. The material kept stretching or the belts slipped through the buckles. Eventually about the third year the corners on to were gone.
We padded the corners but the corners had all kind of green algae etc on them.
A neighbor had his plastic wall paper slip, in a fairy expensive Fifth wheel and fixtures droop and other problems from heat buildup and moisture. The winds were loaded with moisture and up and under the cover.
The newer ones with the wind panels, or whateever they call them should help a lot The only logical effective solution is a building.
That $400.00 cover or less is not a long term solution and in many cases a problem instead of a solution.
Presents a problem where you live, at least with the picture, I have in mind.
Still the best thing to do is a building with air circulation reduction in inside heat from shielding from sun, and protection from all the elements.