I had the same problem on my 5w. I discovered the vent was crimped. The air being displaced in the tank by water needs some place to go. If it cannot escape, water will rush back thru the only opening and make it appear that the tank is full. You will probably find that your vent tube from the tank is disconnected from the vent hole by the filler or there is no tube at all.
My 2016 26rr had a 90* screwed into the fresh water tank with 3" of hose hooked to it. Long story short. I ran a hose from the tank 90 to the tank vent above the filler cap. There was sealer goop in the in said vent..
We have the 241QBXL and had the exact same problem. I simply plugged up the overflow tube and now the tank fills completely full. We've been using it this way for nearly 2 years now without issue and we always travel with a full tank.
Oh Oh. On my new 5w i noticed I ran out of water quickly. Maybe my tank didn't fill it up all the way? It called my attention that water was coming back from the vent after not too much time filling it up....
I had a similar problem. Filled my water tank until water was running out of overflow. I checked the tank and it was full. When I got to my campsite I checked again and tank was only 1/3 full. I thought I had a leak, but couldn't find any. Does the overflow hose go down in the tank and is it possible my water was siphoned out?