Karalyn, the water heater may be located under one of the dinette seats, although the freshwater tank is usually located under one of the dinette seats, and the other is generally storage. That may not be the case in a Casita
though. The valve(s) will be located on the back of the water heater. The water heater can be located inside, of course, by finding the location along the trailer outside, and then looking in the interior cabinetry that covers that location. Once you've located the plumbing on the back of the water heater, the valve (or valves, there may be up to three) should be easy to find. They may have single levers, or water valve style handles. They may be plastic or brass.