William,
The fresh water tank is located between the frame rails about mid-coach. If you have two slides it is accessible from inside the basement, however if you have more than two slides you access it from under the coach.
The InstaHot should be using the same water supply that the icemaker does and the cutoff for it is located just before the icemaker filter that is normally located in the pump compartment on your coach. If the icemaker is working and not the InstaHot, you will have to trace the line an see where it is disconnected and plugged off.
Gerald