Are you sure it is not full already? I know, dumb question, but if your pump check valve is leaking, your tank can fill itself quietly the last time you were connected to city water supply.
Assuming that isn't the issue,
You've got about a 1" I.D. hose running from the fill port to the top of the tank. If it is installed with a low spot, a bubble of water can lay in that low spot and cause back burping on you. That hose should be installed and supported so that it drains toward the tank, but many times they are not. You can try to support it properly. Or, what I did was figure out how far down the low spot was, then took an old white water hose and cut off one end so that I had a piece long enough to slip down the fill spout past that low spot. About 2-1/2' was what worked for me. Connect that to my regular fill hose, slip it down the pipe, and no more burping.
Some of the connectors that attach directly to the fill port work well too. That's what I use now, even though I no longer have a low spot. The connector has its own vent and a nice shut-off valve and works really nice.
Basically you're making this thing, only longer:
http://www.amazon.com/Camco-40003-Water-Filler-Shutoff/dp/B0006IX850/ref=sr_1_2?ie=UTF8&qid=1406152570&sr=8-2&keywords=rv+fill+hose
2006 F250 Diesel
2011 Keystone Cougar 278RKSWE Fiver