2010 and 2011 Winnebago Vista 30W Owners

I conducted the following experiment with the fresh water tank today. I am at campsite and the moho is perfectly level. I filled up the tank completely to the top until the water started to come out of the overfill line. I wanted to check for possible leaks on the side of the tank where the fill and drain lines are located. No suck leaks were found, however, water from the tank continued to drain from the overfill line, keep in mind I immediately turned off the water when the overfill line started to drip. As it turns out, the water continued to come out of the overfill line until roughly 2/3 of the tank was empty. The tank is 7" tall and 4-1/2" (64%) of water ended up being siphoned out by the overfill line.

I refilled the water tank to about 1" from the top, roughly where the bottom of the refill lines is located and no water was siphoned out by the overfill line. I assumed all along the water in the tank that was lost through the overfill line was due to the sloshing around that takes place while driving. Now I know that's not necessarily the case.

When I leave tomorrow, I will refill the tank to 1" below full to see if I lose water traveling , and how much.

I'm stumped why the overfill line siphoned off 2/3 of the water tank. :h