Do what you can to avoid driving through Minneapolis-St. Paul right now. 
Before you turn the key the first time, know what the clearance height on your truck is. Watch the road signs for low clearance markers; you’ll (almost) always have plenty of time to find a new route that doesn’t take you under a low bridge. Having to reload your truck solo into a new one because you gave your old one a buzz cut will ruin your day.
This (mostly) won’t be an issue on the freeway, but rather around towns. Just be aware of it and you’ll be fine.
I remember my first solo long distance trip…I was convinced I’d end up spending the night in a truck stop parking lot somewhere fighting off creeps while i tried to sleep in a freezing car. Of course it didn’t pan out like that, the drive was fine, and I made it in one piece. This country is beautiful; relax and enjoy the drive!
Spend some time adjusting your mirrors properly. Push them out far enough that you can barely see the side of your van. That way you will see more of the lane beside you. I get mine set so I can just barely see the side of my truck and then push them out just a hair more - maybe a bit disconcerting at first, but this gives you a bigger view of what’s going on beside you.
Even though you have a GPS, take an atlas with you. Study the atlas before you start up in the morning so that if the GPS fails or starts giving you wonky information you’ll have it figured out before you find yourself in Calgary.
By the end of the first day, you’ll be fine. A 12’ truck is really just a big-ass van. I’ll agree that you probably should avoid backing up if possible until you’re comfortable with your side mirrors, and you should memorize the height of the truck. Then actually read the “Clearance - 7 feet” signs. That won’t really matter on the highway, but in gas stations it might.
Other than that, if you’re not comfortable driving it in local traffic, you probably want to look for motels near the freeway with restaurants you can walk to.
Other than that, it’ll be fine! Really. Before you know it, you’ll be thinking about taking a job as a big-rig driver.
I forgot - depending on the time of day - if you are very uncomfortable with having to make snap decisions on changing lanes while going through major cities, take the roundabouts. The freeway merges do get closer and more complex while going right through the city. It’ll add a bit of time, but save adding some gray hairs.
Frank gives good advice. GPS’s tend to go the “shortest route” and many times it will direct you through the middle of a major city instead of around it. They are also not infallible. We had several times where we had to adjust because of construction or some other nonsense.
Good luck and keep us posted. Will you have internet access en route? We’d love to follow your progress. 
Clearance.
My mom drove a U-Haul across the country many years ago. The high point of the trip was when she knocked down a low sign at a motel.
As others have said, you won’t have issues on the highway, but you will have trouble at the drive thru.
No, unfortunately, no internet. So I’ll also be going through withdrawal.
All good advice. Except, I don’t know what a roundabout is in this context.
You’ll be surprised how little you’ll miss it. Some state-run pullouts on the interstate do have free wi-fi, however.
I think you mean you are driving a truck with a 12 Ft Box. I rented a 16 Ft. Penske and have a few suggestions.
Make sure the mirrors are adjusted and you undersrand the lighting system before you depart.
(Do you know how to shut off the interior light in the box?)
Keep in mind the mirrors stick out about a foot and a half beyond the box so allow plenty of space on the right for trees, mailboxes and stuff like that (I hit a couple of things)
Backing up is going to be hard because you can’t see out the back. Try to park where you can get out by moving forward. This pretty much eliminates most parking lots.
Those bucket seats don’t recline so the kid might have trouble sleeping if he get’s tired.
Rest stops are fine, use the Truck area not the car. You can pull up to the front of the slot and drive right out.
Have fun and keep the cell phone chatged.
Come on, this is a GREAT ADVENTURE!
Think in those terms.
New landscape, new foods, new landmarks.
I would suggest getting up early, driving until about 3 or 4PM and then finding a nice motel somewhere near a larger city and relaxing with some TV and a pizza delivered to the motel room.
Take lots of pictures (well, have your son do it) and make this the most fun adventure the two of you will ever have! Truck schmuck…after a few miles it is no real difference than driving any other vehicle. By the time you get to NC, you will be considering buying one for yourself - you’ll feel safe and strong in that big MF and maybe want to get a tattoo and hang your arm out the window, start smoking Lucky non-filters and scaring the bejezus out of hitchhikers.
You’ll do fine.
My guess is, the next thread you start when arriving at your destination will be “My Amazing, Wonderful Trip Across Country!”
Oh my og don’t do it!!! You’re gonna diiiiiiiie!!!
Sorry. I’m feeling perverse this morning
Seriously, you’ve got nothing to fear but fear itself! You will feel so proud of yourself when you’ve pulled this off. The first time I ever pulled a camper is when we moved from Michigan to Tennessee. A shade less than 800 miles! I had a couple of white-knuckle incidents in a couple of construction zones, but I did it.
You can, too. Just make sure to let us know when you leave and again when you get in so we know you’re not dead.
Fear itself, yes.
And Bigfoot.
And Werewolves, ooooh, gotta watch out fer them.
You’re gonna be fine.
And had I known with enough time, I’d had said screw it and flown to Tucson, seen a couple of people and driven back with you. What’s two cross country drives in one year?
I don’t know what route you’re taking, but I actually enjoyed the west TX part of my drive this summer. I spent my first evening in Van Horn, TX, just past a border patrol station (the Hampton Inn on Frontage Road there was clean and safe). The roads were smooth, not crowded and easy driving. If you take I-20 to…Dallas, I think, you can head up to Little Rock from there and pick up I-40. The best decision I made on my drive was to go through the DFW area on a Saturday. Traffic was still heavy, but I didn’t have to worry about rush hour.
Have fun and congrats on the move!
A freeway that circles the city. 270 here in St Louis, the Beltway in Washington.
A dozen? Take I 10 to I 40. Turn right. Take I 40 almost to Asheville. Turn left on I 26. Bazinga!
That’s going to be tough, since AFAIK I-10 and I-40 never meet.
Fine. Take I-10 to I-25. Turn left. Take I-25 to I-40. Turn right.
Sounds like half of Thelma and Louise.