I had a very similar experience. Began to occur after I had filled the tank and then towed the camper. I eventually cupped my hand around the water inlet and basically pressurized the tank until the vent line spit out water and then there was a sor...
Baker Park Reserve which is located about 16 miles west of Minneapolis. It is a very nice park with large sites and several miles of walking/biking paths.Clicky
The Dells area is ok, but its a huge tourist trap. I recommend Peninsula State Park in Door County with a day trip to Washigton Island. It books up fast so you'd have to book now for next June.