The pump should be within a couple of feet of the water tank. Turn it on, open a faucet, and listen for it! Between the pump and the fresh water tank, look for a two-position valve, with a 24"+/- piece of rubber hose attached. This is where you add anti=freeze (after switching the valve from FW tank to rubber tube suction).
Before doing that, look behind the water heater for three valves. The top and bottom ones you should close, and open the middle one. This isolates the water heater. Don't forget to DRAIN the water heater, or you'll freeze and crack it.
I drain and blow my system COMPLETELY clear of water with compressed air, then I pump anti-freeze in. That ensures that there are no low-sitting pockets of water.
2014 RAM 2500 Laramie 4x4 Quad Cab w/Cummins Turbo Diesel, 2009 Rockwood 2607, and 3 'yaks!
300W of Suntech Solar, a Rogue MPT-3024 Controller, and a Xantrex PW2000 Inverter