Climb under the trailer and find the fresh water tank. Check that the drain valve is where you want it (open or closed). Follow the fill tube to the side of the camper. Check that all is clear of blockage. The pump does not need to be on for filling. Use low pressure (don't turn the faucet full on). Make sure water is getting to the tank. Check the drain valve again. Is it open and draining as water is going in? Close it. Check for other leaks. Once water is in the tank, turn the pump on and see if water will come out of a faucet.
You might need a new fill tube. On mine it is a plastic tube that I could get at Lowe's if needed. Or check with your local RV dealer.
Make sure you sanitize it. I use chlorine dicor (I think that's what it's called) It is for all intents and purposes, bleach. For bleach, I can't remember what the ratio is.
Have fun with it..