Get the manufacturer and the model number off of the heater. (Probably Atwood or Suburban.) Go to their website and, hopefully, you will be able to download the manual for your specific heater. Yes, if you have one installed there is a sort of a cutoff valve - it is called a water heater bypass. Located at the back of the water heater, it is used to divert water around the water heater so that you can drain the heater to winterize it. You might have to dig to find it.
Happy Camping! ///// Richard D.
2006 4x4 Ford 250 SD / 2007 Flagstaff 827 FLS
One very patient wife and one furry child who travels with us. Forty-two years of trailering and camping, and I still have a blast.