Find the drain valve for the fresh water tank and make sure it is not open. It is usually located outside the RV on the exterior wall adjacent to the tank which makes it an unlikely place for a leak that appears under the RV. If it is closed take the sofa off the wood frame if your water tank is underneath it. There are wood screws through brackets at both ends that are hard to see that attach it to the wood frame. A drill with a magnetic attachment and long phillips bit will give you enough length to reach the screws. When you remove the sofa you will be able to inspect the tank and lines for leaks. If it only leaks when you are filling it up it could be that the filler line is cracked, which can be replaced. You can purchase this line at an RV dealer.
The water pump should only turn on when the water is turned on some where, even though the power switch is on. When the RV is not plugged in the water pump runs off 12 Volts.