My rig is older, Forest; but, I doubt the basic design has changed much. There is a spring-loaded, poppet-type check valve in the housing of the city water fill fitting. So flow is allowed only in the "fill" direction. I just replaced mine due to a leak. I bought one at Camping World (handy and quick) but I really didn't like the replacement much. My old one was brass, the CW one was plastic (and it leaked at the threads on installation). So I bought a brass one and it worked fine (so I just ordered another for a spare). The fitting on her rig is likely 1/2" pipe thread but some are male pipe thread and some are FPT. Someone needs to look. A real easy job to replace and relatively inexpensive ..... though some folks take the valve apart and just replace the O-ring at the poppet.
That check valve has to work if you want to use the on-board pump........or the pump will dump all the fresh water out the city fill fitting.....eventually. No real problem (functionally) when hooked up to city water and not using the pump (although it likely violates some code.) The engineer in me says there should be a shut off valve downstream of the city fill valve so that when it leaks you could isolate it and continue on your merry way using on-board fresh water with the pump; then fix the check valve problem later. (I'd add that feature to mine but access is "turrible".)
Hopefully someone with your daughter's exact rig will respond with the precise configuration. I love having stuff delivered to my door so I got my (good one) here:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
Good luck with the fix............... and safe travels.