Also, check the fuller tube height from the ground up. Level ground. Then check the fill flange on the tank. The filler tube on the back needs to be higher.
I ask my neighbor about this since he has one like yours on the back and he came off the side of the tank and put in a new Fill door on the side of the RV.. He just left the old unused one there. He put it as high as he could above a compartment door. The hose and vent are inside a compartment and are higher than the tank. The hose is right at 3 foot now.