The system (400 series if that's what you have) is rather complicated but if you understand how it works, it can be figured out. You didn't mention your rig make and model so it's hard for me to be specific. As Chargerman stated, make sure the thermostat control is set properly. You can get some hot water from the electric element but for showers and heating, the diesel burner does the work. When I first bought my coach, the burner nozzle had never been serviced and would shut off soon after is was fired up. There is a reflector in there that senses a flame. If it doesn't see it, it won't stay lit. That's what was wrong with mine anyway. I've attached a couple good manuals for you to look at. Read and understand the first one (44_series Aqua hot) and move on to the service manual. I took me a while to really understand it. Good luck!
