Welcome to the Forum!
First, are you saying not filling because the gauge says empty? It's accuracy is questionable at best.
I assume this is a gravity fill tank - no hose connection for filling. Mine is similar. When I try to fill with a hose, the water goes in faster than the air can come out, and it "burps" a lot. I can get water in, but s l o w l y. Reducing flow rate on the hose helps some. What did work was to use a water heater cleaning tool to fill the tank. It is long enough and skinny enough to get into the tank, past the fill hose and fill the tank without the burps.
Even with water in the tank, the pump runs excessively, probably because it has air in the pump and does not prime itself as well as it used to. That may be an issue for you, too.
After trying to fill the tank, have you pulled the fresh tank drain to see what or how much comes out? That may be your best gauge on how much WAS in the tank.