though it's a pain to install, you just can't beat a 110 volt block heater! I find that an hour is usually plenty for a bus, i think they're pretty big amperage draw ie: 1000 watts or more. I put my new diesel vw on a timer to come on about 2 hours before i plan to start my day, but it has a much smaller current draw.
I had a mercedes diesel and installation of the block heater would require removal of the injector pump, so i never did it. I ended up just not driving that car in the winter because it was near impossible to start.
With the exception of my mercedes i've never had a diesel that wouldn't start in cold weather. If not using heat, or not much heat, then i like to put the 100 amp battery charger/starter on the bus before i even begin cranking. This helps her to turn over faster. You don't wanna leave 100 amps pounding into your batteries for very long...so don't turn that charger on until you're ready to begin cranking. I assume most everyone that has a bus has a big battery charger...
other things i've done to get my diesels to start in cold weather.....
small propane heater/stove like this one:
placed under the oil pan for an hour really heats things up quite well. doesn't use much propane either.
i've also used a salamander heater which uses a good bit of kerosene pointed at the bottom of the engine. building a little tent around the heater and the engine compartment on a front/rear engine bus was easy....not sure how things are setup on that mid-engine crown.
and lastly,,,,,there are magnetic engine heaters. They only worked marginally well for me, but prob because i didn't leave them plugged in long enough. They stick anywhere...like on the engine block and the oil pan. Mine were only like 200 watts if i remember correctly.