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You need to press the 12v water heater button or 12v heat button which is in your closet probably above the toilet.
That opens the flow to run in cold weather to engage your heat registers and allow the hot water into the loop.
Press the 12v button.
Did you receive a Marathon book with your bus?
On my 07 I ran with the 120v water heater engaged constantly and always had hot water while under way and when parked that is what you want. I was probably not exercising my Webasto enough.
Later
bv