That's one that can get to most anybody!
So maybe we can start with a few common ideas.
First to really get down to it, we need to know which EXACT RV. In that year and model there was an early and a late build. Early has a 7th digit of 1 and late it 7th digit of serial number of 2. I "think" it the same but worth checking for details.
But a couple ideas to check pretty easy.One is the way the tubing from tank to pump is located higher on the tank than we might like. That means you may need quite a bit of water in the tank to use the pump!
See how high it is shown?
Another easy point to get a mess is the vaccum tube, cap and valve. It is much less work for the pump to move air around than having to lift water! So if we let it get air, it won't move water up to the faucets!
I've marked several places to check, The valve needs to be closed and best to have the cap backl onas well to make sure no air leaks. One check is to take the cap off, turn the valve to suck from the tube and see if you can feel the vacumm suck your finger as the pump runs. Then be sure to to put it back to normal operation!!
If not sucking on the finger, there may be a leak along the line to the pump where it can get air.
One easy one is the fragile little glass filter body that cracks if you look at it! It can also be a filter stopped up?? Washing and soaking in vinegar may help clear hard water deposits.
I lean away from the house filter being a problem as it is on the output of the pump, so problems there might just change the operation of the filtered water, not the stool?? I did not trace it that far, though!
See if that helps and shout back as this is "top oif the head" and may not be correct!
Sometimes giving ideas from a distance is like shooting arrows in the dark!
We never know if we hit anything until somebody yells!