Not sure how detailed, expensive, or DIY you want it to be.
You can make one with one of those small solar (sidewalk) lights that they sell at Walmart, and just stick it into a 3/4" piece of PVC pipe that has been put into the ground.
A "Tee" in the upper section would allow you to put a horizontal pipe, with a sign hanging from it.
All told, you could probably make it for about $20-25. Of course, you can also get more elaborate, and "supersize" it, but that is up to you.
The nice thing with the PVC, you don't have to glue it together, so you can pull it apart to store it in the RV
You can also get a piece of 1/2" rebar rod, to drive into the ground with a hammer, and then place the 3/4" PVC over it.
Happy Camping:)