Under your bus there is, or was, a diesel fired, engine coolant heater. A boiler, a water pump, a fuel pump, all in one metal box/shelf. About three foot long and weighs about 100lbs. This can be used to heat the bus and/or the engine. With that panel, I think, you can set a timer for start up, raise or lower temp settings.
Burns about 1/2 gallon, two litres, of diesel per hour.
If this works you will likely not have to plug in a engine block heater for cold weather starts. New webasto, for the size I expect to see in a bus like yours, $3,000