Congratulations on your "new"
Burro. we have had our 13' Little Mule for about 2 years and it has brought us many good memories already.
Our '87 Burro is insulated. Since I've never owned a non-insulated Burro I have no definitve proof of how much a difference it makes but my wife and I did camp in it last year on a couple of nights in the high 30's and we were about to keep it at 68 degrees with a 425 watt heater.
Insulation was an option on the Burro, so if the original customer ordered it, you got it. Otherwise...
As for insulating post-assembly, because of the double-wall construction I imagine it would be one heck of a challenge. But as Roger mentioned, some have managed to get relectix slid in...
Michael