Black soot removal from refer

[apache3100]

I ran the refer while towing and did not have the fan on. Couple that with no maintenance on refer since new and black soot got inside the TM. Having no success removing it. Have tried vinegar, denatured rubbing alcohol, regular alcohol, simple green and lacquer thinner. Tried lacquer thinner on inconspicuous area and it seems to work but softens the surface and I am afraid it will remove the finish. Any ideas? This is on the plastic/metal? inside walls above/behind bath not the vinyl coated inside walls above/behind kitchen cabinets.