Mine will do that. Do you have a hose that goes back up to the gravity fill spout? On this trailer and my previous one they just put in the vent tube and left it, no hose! So the sloshing would always be spilling out water. With my previous trailer I bought a piece of clear hose and ran it from the overflow/vent to the vent at the gravity fill. Never lost any water after that. On my current one I haven't done it because I don't need anymore than 5-7 gallons for a couple of flushes and hand washing. If I need to I fill up when I get to where I'm going.
