Yes, my vent is the same as yours, mesh taped over a small plastic water line.
I will take good look, cut off a small end piece and use it as a pattern to buy a valve at HD. I will install the valve where I cut off the hose and redo a small screen on the open end.
If I understand you correctly, I will leave the valve open during water tank fill, after water starts to run out of it during a refill operation I will shut off the water supply hose, shut the newly installed valve and leave it closed till I start to use the water tank.
With luck I thus interrupt the siphon and have more fresh water in the tank. No wonder I was running low on fresh when I still had gray and black water capacity. Thought I was seeing some sort of miracle there or bad sensors in the tanks.
I've been RV'ing now since October and am a total newbie but love the learning and really appreciate how nice folks are in the various forums I've read from front to back.