That is a tricky one. How might one tell if a surface is aluminum?
A magnet will tell you it is not steel (possible exception of some stainless steels).
If you could prick it in two places without damaging it with the pointy leads of a multimeter, you could measure the conductivity (obviously if it were some sort of plastic or fiberglass it would not conduct).
I know! Tilt your camper about 45 degrees and get a flat disc magnet and a equal mass of non magnetic material with the same coefficient of friction. If the surface is aluminum the disc magnet will slide slower than the other non magnetic disc because of eddy currents induced in the aluminum.
I knew there had to be a simple solution!