Dr Steve -
Before you get totally into the fiberglass (hope you haven't bought it yet!), do you have any 18-wheeler repair facilities in your neighborhood or nearby? Especially ones that repair the reefer trailers?
If so, go talk to the shop foreman. See if he can put you in contact with the guy who comes by and spray foams the trailers after they've been repaired. Try and schedule a visit with your coach's interior all prepped so all he has to do is shoot it while he's there doing some trailers. Might also be able to get him to shave it smooth after shooting. Win-win.
More expensive, yes. But THE BEST for our applications. And you, living that much closer to the Equator than the rest of us, really need to have good insulation in your coach.
This is one area of your conversion that you shouldn't peso-pinch!
FWIW & HTH. . .