Except for a very small amount of water around the seal, an RV toilet is basically dry. If you want water in the bowl, you add it with the foot peddle before using it. When you push the foot peddle to flush, all the water and contents goes down and you hold the foot peddle until everything is gone.
But, they are designed to not have water until you push the peddle. The do not operate the same way as one in the house. There is no tank to cause a flush, since all the contents simply drop out the bottom and go directly into a holding tank. You use only as much water as you need to clean the bowl out.
These are similar to a toilet on an airplane, except they don't use suction to vacate the bowl. They rely on gravity instead.