The Webasto heater fitted does not seem to have direct heat into the cab. I believe it is the type that heats the engine coolant and this in turn acts like the engine is running and heating the coolant through the matrix and then into the cab. This has two actions, one it heats the cab and two it brings the engine temp up for cold start.
The cab blower motor stops when the ignition is turned off under normal circumstances and will not flatten the vehicle battery, but with the Webasto heater on and the ignition off, once the Webasto has run for a few minutes the cab blower motor can be used. From what I’ve read so far it seems that the blower motors feed switches from the vehicle battery to the leisure battery, that’s why I would like to know the draw on the leisure battery during this operation.
Hope this makes sense
Alan