fresh water won't fill

I had that happen on one of mine. I never really tried to hard to figure it out, I just took my time and let it burp every now and then.

The typical fill port has a small vent hose as well as the large fill hose. See if that vent hose is open and not kinked. Try filling by pushing a hose down into the port at least a foot.

Tanks have to vent in order for water to go in and be pulled out.

If vent line and/or fill line are kinked........tough to get water in

Check routing of both and make sure they haven't fallen down (creates a water trap) or kinked anywhere.

Have you looked inside? A recent poster reported that his overflow line was disconnected and water was pouring into the camper.

I had that happen to me once and it was the vent line that was filled with water due to me overfilling the tank the trip before. I had to blow the water out and it was back to normal.

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

I have had that issue. For me, the fill tubing and vent tubing were both too long causing a dip that trapped water in them. In my case both the vent and fill lines were restricted because of this.
It was a simple fix, just had to cut and shorten both.

I cut about a 24" piece of 1/2" garden hose and connect it to the supply hose. I run the 1/2" hose down the tank filler. The water doesn't have a chance to back up and the small 1/2" hose allows room for air to escape. Now when it overflows, I know it's full...

Just another one of the many wonders of common sense and ineffective construction that gets by as the "standard of the industy".

The tanks should fill on the TOP, not the upper side. And the vents should be attached at the TOP, from BOTH ends. The vent lines should NOT have any dip in them as they exit the trailer. And the pump intake and drain should be on the BOTTOM, not the side. This is pretty basic stuff and they get it wrong consistantly by the thousands every day. Sure would be nice to be able to use ALL the water capacity they advertise for a change.

I have to fill mine at a slower speed or the air can't get out fast enough.