There is a good chance the back flow vacuum break has failed. That seems all to common in RV's. I can't say where yours is located, but often they are behind the shower. If that's where it is, you usually gain access to it by removing the shower valve set.
Perhaps someone with your model will chime in and tell you where it is.
The routing of the back flush usually goes from the hose connection then up a few feet to the vacuum break, then down to the black tank. If it's not the vacuum break, then you may have to pull down part of the underbelly, or maybe cut a small inspection hole to look for the leak.
Here is a picture of what it looks like.