locate the water heater, at the plumbing end (inside your unit) you will notice a loop of pipes with shut off valves that will, depending upon position, allow water to flow through the water heater or bypass it. The direction that the handle is pointing indicates the flow of water through the lines. Follow the flow from your incoming source (fresh water tank or city water hook up) and make sure the valves are positioned to allow water into the tank.
Daniel & Donna Leonard
2011 Ram 4500 SLT 4x4