As mentioned, it could be one of the two possibilities, check valve failure or the Anderson Valve went bad. In June we had our Anderson Valve go bad in Nephi, Utah. I had been carrying a spare since the National Rally in June 2019 since the company said, "these are going bad."
Problem is if it is the Anderson Valve and you want to replace it, it is a 6-week wait for the new valve. The new valve is about $170.00.
Brass Ball Valves, Brass Manifold Systems, Check Valves, New Applications and Retrofit Applications.
www.abcorv.com
I had luck with our Anderson Valve, I turned off the water, release the pressure, slowly rotate the valve right than left 2-3 times. Luckily it would reset the O-rings. This worked until the last time in Nephi.