Can't help you with the Webasto but the simplest way for a temporary warm up on a cold pan is a mag mount block heater like this
Don't waste your money on that magnetic heater. I put one on my bus and it lasted about 2 months till it corroded and shorted out. Big sparks. Thought it was going to burn down the bus!
Buy an engine block heater element and install it in the side of the block. Simple and effective.
TEMPORARY - look it up - it means occasional use. Once or twice a year it will work just fine. Stick it to the bottom or side of the pan for a couple of hours and pull it off when the bus starts. If you need an actual block heater for daily starts that's a whole different matter and nobody in their right mind would consider using a mag mount for that purpose.