It is all about heat transfer more than flowing through one or the other. The Aqua-Hot glycol and the engine glycol are two seperate systems that merely exchange heat one to the other.
The Aqua-Hot glycol system heats up in the boiler tank and has a loop that runs around the boiler tank that contains the "engine" glycol, similar to the domestic water loop only smaller. The heat is transferred to the engine loop from the boiler tank when the block heater switch is turned on and the circulation pump is activated.
When traveling down the road the opposite heat transfer takes place, meaning the engine glycol is circulated through the Aqua-Hot engine loop which transfers heat to the boiler tank and thus raising or maintaining the boiler temperature to allow the heat registers to continue to expel heat throughout the coach.
All that said and out of the way, we too are in the camp that we will not be without the Aqua-Hot system on a coach. It was on our "must have" list when we went looking for out current coach and we love it. Although I had to do some work on it when we first purchased the coach due to the previous owners neglect it has served us well ever since.
I have a friend who has one in a late 80's Newell and it is still functioning properly and he does stay current on the maintenance so they will last quite a long time when properly cared for.
Mike.