I had this very same issue. It most likely occurred because I wasn't paying attention and ran my pump while hooked up to city water. This caused the pump's check valve to stick. You can see if it unsticks itself by just letting it fill the fresh tank and then turn off the city water and run off your pump. When the fresh tank empties, turn off the pump and turn on the city water again. It may work fine, if tank is still filling. Repeat the cycle while you working on replacing the pump.
It is repairable, however, its easier to just replace the pump. My unit is a 2016 bought in March 2015 and the pump issue occurred in summer 2017. I sent an email to the Forest River customer support (you can do this online at their site) and got a response within 24hrs. They shipped me a new pump and did not want the old one back. It came with instructions plus there are a lot of youtubes on how to make the change. Basically, its an easy swap out. It took me about a half-hour to do it.
In my case, the pump was behind a panel in my front storage bin. A few screws removed and you can then access behind the panel. You'll see the pump has a couple of hoses and a couple of wire connections (just take a cell phone pic before you start pulling it out). I unplugged the old one, dropped in the new you, looked at my pics and re-attached the hoses and wires.
I kept the old one to mess with. There's a youtube that shows how to take them apart and in-stick or repair the check valve and someday I'm gonna do that (my wife says stop collecting junk you never get around to fixin').
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