Some possible solutions
First, congrats on your "new" trailer -- used T-139s are hard to find!
Second, some questions -- do you have water in your fresh water tank? In that model, I think the tank was under the left dinette seat.
Assuming you have water, does the pump run at all when you open the kitchen faucet? It will make a loud humming noise. If you hear the noise and no water comes out, close the faucet. Do not let the pump run dry for more than a few seconds -- it could burn out.
If you do not hear the pump at all, are you connected to a power source?
If you have power and the pump is working, check the valves running from the fresh water tank to the pump. Usually (but not always), the valve is open when it points away from the tank and is closed when it is perpendicular to the tank.
Let us know how it is going -- don't give up. All of us were once "newbie" campers, and I did not have any manuals for our old 2003 T-139, so I know the feeling of being lost and clueless. You will soon be an expert!
And yes, do not run the water heater till the pump is working.
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