I was assuming the 2x4's were standing on the narrow part so they could possible topple sideways. If they are going to lay flat I would say 1/4 inch ply is more than enough. If you want the 1/2 inch extra lift you could box the 2x4's top and bottom with the 1/4 inch ply to make it more rigid when adding or removing .
You can also probably go with 2x3's and 1 top coat of 1/2 inch ply if they are going to be laying flat. I went with the 1/2 inch in this suggestion in case the camper walls are thicker than the 2x3. In that case the extra thickness of the 1/2 inch would better spread the load.
Depending on wall thickness can you get by with 2x2's instead?
