I'm not sure if this point applies to the 400LP in the latest Apires or not. But if you have a complete loss of controlling 12 volt power to the Aqua-Hot system, first check the 25 amp fuse in the front electrical center box outside below the driver's window. It is the the lower of the two fuse panels. If it is good, then open the Aqua-Hot unit and check the upper 20 amp fuse on the higher left interior side of the box. I will say that Entegra CS as well as Aqua-Hot CS came to my rescue with a replacement circuit board that got cooked thanks to a metal drain valve in the Aqua-Hot that came loose. Why do these things happen on a cold weekend day?
PS: If you do need to replace that 20 amp fuse inside the unit, do it with a 10 amp fuse. It is better to stop it there than farther up the line such as your central control board.