Or ... instead of ceramic coating the exhaust ... merely install metal heat shields right above the exhausts to keep the hot air from rising up to heat the underside of the doghouse.
Our Class C's E450 chassis came from Winnebago with heat shields between the cab floor and exhausts, and the doghouse has never been warm enough to be of any concern. I'm not sure who installed the heat shields - Ford or Winnebago?
When traveling in hot weather, we can always split some of the A/C's air flow down to the floor using the dash controls.