First: When you say "Backflowing through your fill hose" do you mean "Backflowing through the fresh water inlet?
No, that is not the fault of the water pump.. In fact it's no where near the water pump.
The inlet, be it a straight inlet like I have or a Sure Flo regulator inlet, has a check valve in it.. This check valve has failed.
Now: Some folks warn that "If you do this you can destroy the valve and it will never work".. I think they are wrong but consider this... it is not working now, so if you make it not work, Well nothing has changed. (Which by the way is the origin of the myth,, It's not working now, You can't make it worse).
Remove the screen washer (Pump off) NOTE you WILL get wet in this procedure
Under the washer you should see the actual inlet and there should be a 3 or 4 "Arms" holding a small circle (like a washer) with a button in the middle.
Turn on pump, water will flow
Push button with pencil eraser or short rod
Release
you may need to do this 3 or 4 times
If no success by 5th try, TEMP solution is to screw in a plug, You can get plugs at most RV stores including some campground stores or you can get a standard male garden hose adapter (3/4" to hose or 1/2" to hose) and a matching pipe cap at any hardware store.
Long term solution: Replace inlet.. USUALLY this is a very easy job.. If the inlet is mounted in something with screws holding it in remove the screws,, PULL the inlet and benzel toward you gently,,, Hoses and pipes should have enough give, Unscrew by holding nut on back where plumging hooks to inlet (or holding elbow) and turning inlet counterclockwise.. Take to RV store, get replacement, replacement is reverse of removal.
Alternate repair: (I rather like this version but..) Most hardware stores will have a garden hose size "Spring check" valve (3-4 inch you may need brass adapters) screw it into the outlet, job done.. You may want a short strain relief hose to make hookup easier.. Or a quick disconnect system.
