mounting a portable gen in a gen housing on TH - why not?

My wife's cousin did exactly this when his onboard died in his MH. I an attest that it works as I was parked close to him and he ran it ALL NIGHT LONG for his CPAP. The one he has is NOT as quiet as most onboard units, not sure what brand it is. He did add an external muffler/spark arrester off his son's CR80 so exhaust is quiet but it's the mechanical noise that's loud.