I believe it's a 1997 Leisure Bay. Just installed in deck and no heat. Inside the main electrical box there is a small terminal block with screws for each of the wires, which apparently is used as a junction for the interlock, hi limit, flow and thermostat swithces. I believe they are all in series with each other that eventually ends up at the heater relay. Each has four wires at the block except the thermostat which only has two. I checked with a volt meter through each of the components and I have voltage through all up to the thermostat "screw". It seems that there should be two more wires connected to the other side of the two thermostat screws that go to the thermostat. Just for kicks, I jumped across the two empty screws, the heater relay activated and the heater came on as did the heat indicator lamp near the thermostat. It never shut off. The water temperature was very hot and adjusting the thermostat knob did absolutely nothing.
Are there wires missing? Please help!

by jumping the two wires on the thermostat you have effectively told the system that you want it to heat ALL THE TIME. If the system never heats with these two wires separated then more than likely you just need a new thermostat to fix your problem.