I don't have knowledge of a Mandalay, but I would think it is in the same compartment where your fresh water fill connections and selector valve are located. Just open your outside storage compartments, and listen for it.
But the fact that it is constantly running is not an indication that it is bad. It will keep running for a number of reasons, such as a clogged filter screen (at the pump), blocked pickup tube, a leak preventing the system from pressurizing, and probably more often than anything, the "City/Tank Fill" selector is in the wrong position. Check those things out before you replace the pump.
USAF 1965-1971; USCG Master
2002 Horizon 36LD - 2000 GMC Sierra 1500 4x4
Indian River, De