Can't fill fresh water tank to capacity?

[FirstTracks]

I'm still learning our first TT, a 2016 Evo T2050 which according to specs has a 56-gallon fresh water tank. It's a gravity fill with a small screened tank vent next to the gravity fill cap.

No matter how slowly I fill it, I can't get even close to 56 gallons in there. I'm using one of those Camco water tank fillers attached to the end of the hose, which has a valve to let me adjust the flow to a trickle, and I've placed an Orbit Water Flow Meter in line to monitor how much I've put in the tank.

Today, I started with a drained tank to be sure. At around 28-30 gallons the vent started burping. At around 36-38 gallons the vent stopped burping. At 42 gallons, water began to flow from the gravity fill, even when the hose was barely dripping. That flow continued even after pulling the hose, and it seemed like as much was coming out as was going in so I stopped even though theoretically I should be able to get another 14 gallons in there. But another 14 gallons isn't trivial.

Is this normal?

[mn camper guy]

Bump for update.
Did you ever resovle this? Had our maiden voyage last week and I think we had a similar issue (vent burping and backflow out opening). Tank ran out before I feel it should have sensor read full.... Old trailer with 10 less gallons would last longer than this one did.
New trailer is a 2018 2509S

[FirstTracks]

I figured that between the 6 gallons for the water heater, which I didn't realize that they included in "fresh water capacity," and the plumbing that they must include too, I was within a few gallons of what should have been expected.