The sheet metal is just a vapor barrier, so exhaust gases do not get into the passenger compartment.
I have seen custom Onan installs where they install a 25 gallon gasoline tank under a fifth wheel or TT. Back in 2011, I was quoted around $4500 for the genset plus the custom fab work for the tank, the generator cage, wiring, exhaust, and plumbing.
As an alternative, the OP might be better off with a portable genset like a Honda eu7000i with remote start. Propane generators are thirsty, and tend to not have much runtime.