I removed what I could with a razor blade and a putty knife, but there was still a thick layer of black, sticky residue remaining. I went to the hardware store and found a product called GooGone. WEARING GLOVES, I put some the solvent on a paper towel and rubbed an ample amount on to the surface of the hull. I allowed the solvent to set for a few minutes before wiping it off with another paper towel.
(Note: I am using very heavy duty shop paper toweling)
The GooGone took off, or kind of melted, most of the tar. The small amount of tar remaining cleaned up with lacquer thinner.
Now to get the latex paint off the fiberglass inside......
I will be posting many Instructables about the restoration of this trailer, next up: Demolition!