You'll most likely find it by looking inside the coach -- under a bed, under a dinette seat, under something. Hint: put some water into it, and run the water pump, listening for its sound. They are usually located close to the tank.
It's most likely on the other side of the wall where the fill is. Could be under the rear bed, built into the back wall. In motor homes there generaly in the living space to keep them from freezing, Bill
I would say also- its near the pump(which has to be easily serviced)your fill and drain lines should also be easy to check and replace when needed. spend a afternoon and lift beds and poke around your RV - just to see what and where all tanks and pumps and connections are located.You may also learn that a kitchen faucet left on just a little will fill the grey tank and then the tub and then overflow and flood the RV!!!!!!!!!!!!-I was lucky -I had a carpet cleaner and spent the night drying the carpet-clean as new now----the disadvantage of a nice quiet shur/flo pump.