You might try a thin layer of diluted paint stripper. Often times paint stripper will only remove one layer at a time... test on a small area.
You might also try lacquer thinner. Sign paint is enamel and old trucks were painted with enamel and lacquer thinner has much less effect on enamel. You may have lucked out and the top coat could be lacquer which could be stripped off quite easily without having much effect on the paint underneath it.
Sandpaper or any other abrasive is just going to cut whatever it comes in contact with.
If it were me, and none of my suggestions worked, I'd go at it with the sandpaper until I could see it well enough to make a tracing and then find an old-school sign painter to re-create it.
Regardless, I'm always looking for vintage door art and would appreciate a photo for the collection linked to below.
