Looking at a layout of your floorplan I would guess the tank is under the closet, under the sink or as Cavie said under bed. Easiest way to find the tank is look inside the coach under the outside water fill inlet.
When the pump is on does it run continuously? Has the coach been winterized?
I'm having a brain fart here but there should be a valve that is closed between pump and tank that allows you to pull antifreeze to the pump and run it through out the plumbing system. That valve needs to be open to allow the pump to pull water from the tank.
2018 ACE 27.2
Toad 2019 Jeep Cherokee Trailhawk
Roadmaster Falcon All Terrain Towbar