Couple of things to check,
1 the connector for city water should have a plug and should not be leaking,
2 is your toilet running on and filling up the black tank?
3 Is you floor wet near the back of the trailer.The tank is probably only 17 gallons but if its empty then you should be swimming in it.
4 If not then check for leaks
under the trailer such as the tank drain.
5 If its just down about 1/4 tank or so then it may just be that your hot water tank was empty and when you turned on the pump, it filled the hot water tank with 6 gallons
If the floor is wet, then you probably have a leak in the pex fittings or the pump ( if you're in a cold area, was it winterized last year.)