This is a great opportunity!!! I'm so glad to hear you got it!
Because extreme cold is a possibility insulation would be a top priority to me. You probably don't have time to do anything nice so if it were me I'd get rolls of 48 inch Reflectix and make a tent covering the whole inside. Usually I don't recommend it but if it's facing your Mr.Buddy heater it will Reflect that heat back in. Condensation will be a problem, you may have to wipe it off.
Drafts will make you very cold so drape Reflectix or heavy blankets over the back door and side doors sealing them so drafts don't get around.
Above all else you must stop the flow of air up to the cab with heavy blankets or cheap sleeping bags from thrift stores. Heat will pour out of the front windows and cold in from poor seals.
Have a good time!
Bob
The Mr Buddy is a good heater but I wouldn't leave it on while I slept. I have slept for 6 years with an Olympian catalytic heater on and trust them with my life.