Originally Posted by Tundra 2014
If on the road, you can get a twist-on supplement valve. It will solve your problem until time to fix.
While it hasn't been clarified by the OP... I don't believe the problem is at the sewer port. (black pipe)
I believe they have a black pipe (hose/tubing) hanging down under the R/V that is the overflow for the fresh tank and water is coming out of it. They mentioned the fresh tank filling so that's why all the suggestions for the pump's check valve.
To the OP... can you clarify for sure?
Always posting as much information as possible and answering the questions asked (see bikendan's post # 2 asking about the hose) will help you get your issues resolved with the least amount of guessing on our part.