Is it possible for the FBH to be wired directly to a leisure battery in the boot of my 2012 L322 Westminster, to allow heat into the cabin on a cold night when camping in the rear of the car. The rear is converted to an overland camping set up

why don't you boost l322 battery with an additional battery when camping
fhb will not work if it has 12v, and the other consumers in the vehicle do not,
first of all I mean hvac/ventilator

Be a lot easier to fit a self contained diesel night heater when you need it. Wouldn’t be too difficult to deal with the flue pipe requirement.
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