Granvil,
The Aquahot and Webesto we have are two different systems. Your Aquahot has it's own dedicated coolant reservoir and our Webasto's are using our engine coolant. I had several Aquahot's in plastic coaches and the electric function would give you hot water, but not enough for say two showers. With your diesel selected instead of electric, the hot water supply is endless.
Our 20 gallon tanks do a great job on electric, are pretty quick to recover and we have never run out of hot water. I use the Webasto for engine and cabin heat, but the side benefit is heated water at the same time. If I'm dry camping, I turn on the hot water heater when the gen is running and that pretty much takes care of the water needs.
The exception to not running out of hot water would be Mango with 4 women aboard, he would run out of hot water even if he had a commercial boiler installed.