Karl,
Maybe I misunderstood your post, regarding preheating. If so, disregard this.
As I understand the system, only the Hydro Hot 200 series has a preheat pump that when activated, (w/burner on) circulates engine coolant through the Hydro Hot to assist in warming the engine. This is an independent coolant loop in the Hydro Hot and not connected to the coolant loops that run to your heat exchangers to heat the coach interior.
The Hydro Hot units w/o a pre heat pump (e.g. model 500) also have an engine coolant loop coming into the unit. However, this is not for preheating as the only way of circulating the coolant in this circuit is by running the engine. Again, this is separate from the coach heat exchanger coolant circuit. Pre heating on these is accomplished by heating the intake air. By running the engine coolant to the Hydro Hot in these models, you can have hot water and some heat when driving w/o having the diesel burner on.
That is my understanding.
Steve