I had same issue with mine and the valve was replaced twice. Then one of the techs told the process for switching to/city water and tank fill mode. Been using it for 2 years now without issue. There is an O ring that leaks if the valve is rotated under pressure. A few rotations of the valve will reseat it.
First turn the water supply off. Then open the hose in the wet bay to relieve the pressure on the supply side of the valve. Next at the bottom of the plumbing “manifold” open one of the low point drain valves (I usually use the cold water valve). This relieves pressure on the back side of the valve. Now you can rotate the valve handle. I seem to recall him saying that if you turned the valve to the left (city water to tank) then rotate in the same direction to return to the setting you want. It’s only 2 positions so what the heck.
I’ve never had a problem since using this procedure, and I’m constantly adding water to the fresh tank.
Fulltime since 2008
2018 Ford F-450 King Ranch with a 2019 Mobile Suites 40KSSB4 “toad”