Often the freshwater fill port is covered by a small locking door about 4 inches square. Open the door and you see a hole large enough to put the end of a garden hose into. Above it is a small overflow hole. You run the hose until water squirts out of the overflow hole.
Sometimes the pump cannot draw water from an under vehicle tank without being primed - particularly when the tank is not full. This means the pump must have water already in it in order to work. In my MH, I have a winterizing kit - a valve on the inlet side of the pump so it can be switched from the pipe coming from the tank to a tube that I put into a jug of RV antifreeze. To prime the pump, I fill the tube with water and stick it into a jug full of water, run the pump until it visibly draws water from the jug, then switch it to draw from the tank.
There should be a freshwater tank drain tap, usually low down on the back of the tank. A neat solution to the unreliable tank level indicator lights is to attach a clear plastic tube to the drain tap, long enough to extend up to the top of the tank and attach or hang over something under the RV. Then you can open the valve and clearly see the tank fill level in the tube. In your situation, you could just leave the freshwater drain open while you try to fill the tank - water draining out will tell you water is going in. Or not.