It is usually immediately adjacent to the water heater, on the back (inner) side. Sometimes you have to remove a drawer or panel to find the valve(s). I'm not a NEwmar owner, so can't give more specific help.
IF, and that is a big IF it is like my 2001 Newmar Dutch Star, there will be3 valves accessable from the compartment next to the water heater. In my case from back to front there is the power compartment, the water one, the heater and a small door just to the forward side of the heater. The valves are in there along side the heater. There are 2 in long runs of pipe and one that crosses between the 2 lines.