We have no online help when it comes to the trailer group so it will take a bit of looking to understand what is happening.
The reason the pump cannot draw water from the fresh water tank is likely a valve letting air in!
The air being so much easier to move than water goes through the pump instead of there being a vacuum created to make water move from the tank.
Looking at the pump it likely has a small round filter that is on the intake side of the pumpas a clue to which direction the water will flow through the pump. Nearby is likely to be the suction hose which has a cap and n connection. This hose with cap removed is meant to stick in a jug of antifreeze and as the pump pulls a suction, it draws antifreeze into the lines.
There should also be a valove to shut off flow from this suction line to the pump!
If this suction line valve is open (normally the handle inline with the tube?) and the cap is left off, air is pumped instead of doing the harder swork of moving water.
Verify cap on and vales set to close the line!
Understand that this pump is self-priming and will run finewithout damage when pumping air, but when pumping air you do not get water!
So if that valve and cap are okay, look further toward the fresh water tank for a valve fo draing is left open.
This drain line will show up as a tube, mostly simply going straight down through the trailer bottom to drain outside. It will likely look like a valve with three tubes going in three directions. Careful when looking as the one going straight down may be missed if snuggled down to the floor!
If this valve is left in drain position, air come up and to the pump, change it!
I made this drawing as a way to see what can happen. The valves "normally" need the
handles set to indicate which way the valve inside is set, but DO NOT trust that too far as some differ!
This is not likely to meet what you really see but look for something similiar. Find the tank if you can and look along the line from the tank to the pump for any valve that would drain to the outside, and close that. As well look further to spot the suction hose diverter valve which will change the way air from outside or water from the tank can move. Close off flow from the suction hose!
You want a solid closed line of tubing from tank to pump with no way to get into things!
It is sometimes as simple as the small fragile filter housing is cracked or broken if they did not get all the water out in freezing temps, so look at it for being screwed on and not broken!