rjohnsonnm, do you retighten after the tires cool down, after setting up? If not that could be some of the reason for the "loosening".
I also use between the tires chocks, but I do not experience that problem. However, I do use "RotoChoks" I have used them for over three years, and I still think they are great for doing what we need. I do put them on both sides. Many, many, many............................good reviews on the varuous forums.
Yes, they are a little pricey, in comparison to others available, but for what you get, and how well they work, IMHO, well worth the money.
http://www.rotochok.com/