Needs a location
Well, I finally found the winning combination to remove the tar: alternating paint stripper and mineral spirits (each one of those alone doesn't work as well as the combo). So that is what I'll be doing at the next build day.
For anyone interested in helping, I will provide PVC gloves (for the stripper), other kinds of gloves, face masks, and safety glasses. In the way of tools, we'll mostly be using steel wool, wire brushes, and rags. I have drills and brush bits, but they're not working as well as elbow grease.
This is very dirty work so don't wear clothes you care about. As usual, I require guests to sign a liability waiver before entering the Airstream or using tools of any kind. Also, please note the time change: this is starting at 11am for this month only.