Very logical thoughts.
BWilson--the drain line for the fresh tank is directly below the Kantleak valve where you are trying to put water in. Behind the water manifold down deep below maybe some insulation. Find it and open it--if water comes out, maybe D is right, tank may be full. Lots of time you will get water burping out that vent while filling--try slowing down the spigot feed control.
Had a '15 RSSA, they are all the same on water feeds.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired