Being we are remodeling our house, we took the siding from our mobile home to use on the back wall of the new room. I was surprised what was underneath the siding. Looks like cardboard to me. I wondered it that was all there was. Then, we noticed places where it was cut, for them to install wiring (?). So, I lifted it and did see some insulation.
So, if you are wondering what may be inside your mobile home wall, look at these photos. Ours was built in 2008.