Jim, Accepting advice from a Vision and a Liberty owner may not be the best thing. Not that these guys give bad advice, but because their advice may be in the context of how their coach is set up.

I haven't studied my present Liberty with respect to bay heat, but on the previous coach which spent 10 years traveling in cold winter weather, sometimes as low as minus 20F I had a clear understanding about whether I needed bay heat. In the case of that bus the answer was I didn't have to do anything because heat in the living area was also diverted into the bay.

I have a heat exchanger in the water bay on my current bus, however I have not looked to see how it is regulated. It may function the same as in the previous bus by heating the bay when the Webasto is working or when the bus engine is running. I also have a thermostatically controlled electric heater that is controlled in my 120V electric panel.

Your Vantare may be set up different than my Liberty, jim's Liberty, or Pete's Vision. It also may be set up different than other Vantare's so it might be safest for you to get advice from a Vantare owner or expert, and then verify that information. Usually I am not so dubious about advice here, but if you start feezing in your water bay and various components bust it can very expensive.

As a FWIW comment the best protection against freezing is a warm water holding tank. I used to keep a 500 watt cube heater aimed at the water holding tank on our first bus when it was parked. That heat output over time warmed up the mass of the water in the tank to 50 degrees or so and despite some bitter cold periods nothing ever froze because that 130 or so gallons of water kept the bay from freezing.