Trick question? Under what circumstances would you be in the coach and the HW tank would be completely cold?
If we are dry camping every time we run the generator the HW tank is getting heated because the switch is on. If it is cold out and we are using the Webasto it is heated as the coach is heated. When hooked to shore power I use the electric HW heater. Our HW tank is 22 gallons and it holds more than enough heat to still give us a hot shower up to 24 hours after we shut down the engine.
My webasto will shut off the burner in about 30 minutes from a cold start and at that point the Hw tank heat exchanger has 160 degree coolant circulating through it.