Might be good to know what TT you have. In general, it seams Jayco cuts all the gravity fill hoses a little long so you have to fill up hill. I have never heard of anyone having water pressure against the gravity fill cap.
I found a Camco water tank filler. Shove it in and I do not get any water backing up against the fill port or leaking out as I fill. The one I have does not have the shut off valve. It works really good.
https://www.amazon.com/Camco-Shutoff...gateway&sr=8-1