I agree with Stu & Filo. T. The tar appears to be intended to hold the wool insulation in place, or some others say it was applied as a rust preventative. I don't believe the coverage is complete enough to work as a rust prohibitive coating. The unsprayed areas of the ceiling had no signs of rust at all. I saw no signs of rust or leaks indicating the tar needed to be removed. I considered attempting to remove the tar because some other people here were removing it from their ceilings.
It's way to cold to scrape off tar from the ceiling in Oregon. It would likely chip off unless the removal was done during the heat of the summer. Otherwise chips of tar would land everywhere and make a big tarry mess on the floor. Personally I take a shortcut when I can, so I had the foam guy just spray over the tar. I don't have any experience to compare this foam job to so it's difficult to say what is the best way to do things in preparation for foam.
I liked the idea of completely filling the cavities between the ribs with foam, rather than applying a 1/2" layer of foam. That issue is what caused me to shy away from the kits and lean toward the commercial spray foam. I'm sure there are advantage and disadvantages to either method. I've only done one method so I can't say which is best. I sure did feel warm in my bus this recent winter, and I put much less energy into staying warm. Worth it.