Re: Winter conversion
All the above is good advice. Your best bet may be to cover the area and wait fro spring. I live in Maine where is is frigid. One thing you could do is enclose the perimeter of the bus with something like hay bales to keep the wind out and run a contractor fan forced propane heater under the bus. This will warm the area almost immediately. You might even consider this general idea to heat the bus in zones as you work. It won't take much to heat a 10x20ft area while you work, then move the heater to the next area for a bit to heat it up. You could probably do this as well from the inside of the bus if you're working in a small area. You don't need the metal heated too much, just match the specs on the sealant. I would also bet you can find a cold temperature sealant that would work in a much wider temp. range.
I personally got all my sealant work done in the summer. But now the majority of my work is done during the winter because that's when I have more time. I heat my 30ft RE Bluebird with a small wood stove vented out the roof. I can get the entire bus up to 70 in about 2 hours and hold it as long as I need for pennies, regardless of how cold it is outside. But I also have VERY heavy foam board insulation on the floor and walls.