If you have a Harbor Freight nearby, they have some great rubber wheel chocks that you can find on sale a lot of times for $5. These things are a lot more sturdier than the plastic ones, which depending on the type, I have actually seen get crushed......and you can use your gift certificate for other necessities.
You won't crush the solid rubber Harbor Freight ones.
Rubber Wheel Chock - Save on this Solid Rubber Wheel Chock
And ALWAYS chock your RV before unhooking from the tow vehicle. I have seen a few trailers roll away because the owner forgot to do this....including one of my camping friends. It bent her tongue jack and ruined a stabilizer, as her trailer rolled. Even when you think you are on level ground, chock those wheels.