I'll add a couple of things. Your box is wired for 24v and 110. Mine had a converter in the box, which it looks like yours does too. If you put 110 energy saving bulbs in the ceiling, it barely touches the battery. I have a 40,000 btu diesel heater in mine, it will keep your toes toasty, and can be used to cook pop tarts (the pastry type, not britney spears). I would recommend that in a minute. Also, make a connector for 110v for your box. You'll see what I mean when you get it, it's the military style and is hard to get a gen to hook up.
I'd also recommend a second set of 2 12v batteries in the box. Can be used as a jump for your truck, and can run your accessories without killing your truck battery.
I have a 24v to 12v converter in the cab of the truck.
Pyle PSWNV720 720 Watt 24V DC to 12V DC Power Converter 267-7340
Money well spent, provides steady 30 amps of power generally, up to 720. I put a 12v cig lighter on my dash wired to mine, runs gps and charges laptop/phones easily. I also put in a switch to turn it off when not in use.
For simplicity and probably cost, I'd go with military winterization kits. Remember your automatic is probably filled with 15w-30, you may want to switch to ATF, AND get a heater for Montana.
The seat in the A3 isn't bad for trips. Unless you're the passenger.
Carry caps for the wheel hub connection for the ctis with you. When they fail, sometimes it just doesn't work, sometimes it drains the tire flat, and sometimes it pushes gear oil into your tire. I think it's a 7/16's NPT cap, but you'll have to check. Carry 1/4 air line, 3/8's air line and 1/2" air line, about 10' of each. You have a plethora of lines that can go bad, and do with temp changes/vibration. Keep fittings for all of the above as well. T's, elbows and connectors, you can often use "gator-bite" type connectors to make life easier.
Keep a gallon of leak detection fluid. (Soapy water in a spray bottle). You're going to need it eventually. Check all your tires, CTIS plumbing and around the top of the compressor right away. You're going to have something leaking, and I'll bet you a dollar right now on that.
Check your fuel/water filter. Actually, all your filters should be replaced/checked. Here's a list of cat parts.
Oil- 1R1807
Fuel- 7E9763
Trans spin on (external)- 1R0713
Air- P520925
When you get your insurance see if you can get towing. Get unlimited mileage, it will pay for itself on the first tow, for your lifetime.
Check your generator bracket. Most of them are weak, and poorly designed. I think C and C has some new ones that will save you some pain later.
Keep a 1/2" breaker bar and a 2 inch extension with you. You'll need it to change the belts. Wheel nuts are 1 and 1/8, lug nuts are 1 and 1/2". See if you can find yourself a nice IR titanium impact wrench if you want to change your own tires.
The truck takes forever to get to operating temps, so a radiator cover might be in order, part of the winter kit.
Refill your ether start bottle, or keep a spare.
replace your windshield wipers now, they are usually bad, and you're going to be driving in snow. You'll need them. Some boat wipers work too.
Enjoy the heck out of the drive! if you're heater is working, you'll be singing it's praises. If not, you'll be cold. If it's raining, you'll be wet. The front window seals are usually bad, run a garden hose on them to check. Duct tape the bottom of the cab rear window before it gets wet. You can duct tape the front windows too.
Speed. The TM's say DON'T fun full throttle for long periods of time. Goodguys is right, 48 is more likely, tops is 52. Maybe 52.1. Probably 52.0.
If you're bucking a bit while driving in snow, hit the front wheel drive. I noticed more in sand, but if the back starts slipping, it tends to buck a bit.
Get a chain to lock the steering wheel. Pad the chain where it goes on the wheel.
Teflon tape. Lot's of telfon tape.
3 or 4 five gallon buckets with lids. For leaks, and seats. Box of shop towels. I keep tools and parts in the 5 gallon buckets to keep them organized. Get some 3/8", 7/16", 1/2" grade 8 fine thread nuts and bolts.
Lastly, have fun! If you get in trouble, drop an email on the site, one of us will help.
Geez, did I miss anything?