If you have an upper gravity fill, with a vent, like I have on my Monaco coach, you can plug the overflow pipe(s) and everything will work just fine, with no lost water. Think about it, the gravity fill/vent eliminates a vacuum being created in the tank so if you leave the gravity fill open when you fill the tank, as soon as water come out of there, everything is full. I just installed a valve on my overflow pipe in case it was a problem to block it. That was four years ago and I have never opened the valve and do not have any water siphoning or running out of the tank once I stop filling.
This works for me but I also watch the gravity fill and am sure that I am there to shut off the water as soon as it starts to come out. With our high water pressure, I'm not sure that the overflow can cope with the 90psi input and I do not want risk splitting the tank or rupturing the plumbing.