Water tank filling.

Do you mean from inside the coach? No. Mine is filled from outside only.

Do you have a water fill valve that needs to be turned?

I use one of these Water fill hose attachment but I extend it by inserting a short length of narrower tubing which I super-glued in place.

gravity fill?

could have an air lock, try using a longer / skinny hose to get farther into the fill port.

less likely, could have an insect nest in the tubing between the fill port and tank.

https://www.youtube.com/watch?v=NBkVwoiR57A

Air bubble......slow down the water flow to let the air escape.