fresh water tank possible vent blockage

OK. Update on FW overflow. The exit for the overflow is on the lowest point outside of the under cover. There is a rubber fitting pushed onto a 1/2" threaded brass nipple. I took the rubber fitting off the nipple and started filling the FW tank but not at full pressure. I filled slowly so the the tank would not expand more than normal when it is full. As soon as the first dribble came out of the nipple I turned off the fill hose. The overflow was a small amount for a few seconds. The panel indicates that the fresh water tank is full. I can only think that the position of the overflow tube inside the tank is way below the top of the tank, approx at the 1/3 level and once the tank is expanded way more than normal because I have the hose bib in full open that when it starts out the overflow nipple it starts syphoning and won't stop until it seeks the inside end of the pipe. I'll check things later to ensure it is not syphoning anymore. Yes all the tank level indicators work and are accurate. When I fill the black tank between trips and winter storage I use a flashlight and can see when the toilet becomes full to the throat. I've been keeping all tanks full when not in use and I think it has kept the sensors in the tanks from corroding. I just checked and there is no syphoning and the panel still indicates full.