OK now I'm a little confused....
No you don't need to have the water pump on to fill the fresh water tank.
Yes you do need it on to have pressure to your faucets and such IF you are not connected to city water line. A lot of trailers can only fill the Tank with a gravity feed hole with a cap on it (some have an optional valve from the city line but lets work on your fill port since that is what you were using).
Sometimes when filling the fresh water tank through the fill port you can actually fill too fast and create a pressure backup. This is worse if the vent line is clogged (could be as it hasn't been used for a while) this line is a small vent line that might go to the roof, could come out under the trailer, or might be a small screened port next to the fill cap. You may want to try the filler nozzle thing linked to above and sometimes that helps or maybe just slow the water pressure down can also help. The fill line might be kinked or have a bend in it that also slows down the filling process thereby making you think it was full. That line is the one from the fill cap and you will have to figure out what is on the back side of that fill port to know where to look to see the hose if you want to see if it is kinked.
The quantity panel may or may not work. Usually they last a while for fresh water but not so good for Black or Gray. You will have to check once you know you got water in the tank.
Personally I would try just filling it up with a low pressure from your hose and see if it takes water at all, and then turn on the pump and see if water comes to your faucets. You will hear the pump when it operates. Usually right after you turn it on until it builds up pressure (a second or two) and then fairly continuously once a faucet is opened.
Good luck.