The problem I have with a lot of chocks from HF and the like, is that they are sized for big trucks and are too steep to work properly with typical RV trailer sized tire. The typical truck tire will be over 40" diameter, and the typical RV trailer tire will be closer to 30". It makes the angle all wrong for effectively wedging our smaller tires.
It would behoove some of you to actually test pulling against your chocks and see how they work. A good chock should work on loose dirt.
Personally, I make my own out of scrap lumber and have tested them to be sure they work.