I keep a 2foot piece 3/8 rubber hose in the compartment where one fills the water. While filling I'll put one end of the hose against the little metal screen above the fill opening, the vent opening, and blow thru the other end a couple of times during the fill process. Sometimes it's clear and sometimes I have to blow the trapped water out of the vent line. PITS but it works.[/QUOTE
I do the same thing each time I begin to fill, there is always water in the vent line that I blow back to the tank.
After clearing I can run my fill water full blast to full.
There has been a good deal posted about this problem. A search will get ya lots of info. I had to undo the fill tube and pull it stright and cut off some of it. Others run a hose down the fill hose. I'm sure you will get several fixes. An easy fix tho.
Get one of those rubber hose attachment used for a faucet that is not threaded. They fit perfectly inside the fill port and applies just enough pressure that it blows the water out of the overflow tube which has water in it because of a low point. This things blows the water right out and fills the tank full every time. The water in the breather hose will not let the tank breathe and that is why you cannot fill it. Have used this method for two years now without an issue. This link will show you a picture. You can buy them at any RV supply place, most dealers. Before using this I had the dealer try to fix it and it just kept happening until I discovered this.
http://www.campingworld.com/shopping/item/water-thief/4690