I've been going on the road with my man just as passenger since September 2010. I don't go out every time, but frequently. Here's the facts that I felt I had to keep in mind from the onset:
1. Space is tight. Either deal with that fact or stay home.
2. This is his line of work. It's not a vacation. Fun can be had, but work is first and foremost.
3. Since it is work, there will be no motels, no restaurants; must keep overhead down.
4. Stay out of his business. He runs the show, it's his livelihood. If he wants my opinion, he'll ask for it. Not my baby.
5. Help when I can; organize paperwork, prepare the food, keep the van (or now, truck sleeper) organized, make room for freight in the van's cargohold, keep track of fuel prices along the route.
6. There will be encounters with bitter cold weather, sweltering hot weather. Oh well.
7. Bear in mind this is really a good quality time experience. A lot of topics get talked about when you're in the vehicle in the middle of the night SB I-55.
8. Truckstop showers are not the end of the world and they won't happen every day.
I was a great distraction from the waiting game back in his expedite days. Because of my presence, we'd go explore the area we were in, because that's more fun to do with someone else than alone. Even at night in the truckstops, someone to talk to and people-watch with. It takes a lot of selflessness to be the passenger in these situations. The ability to go with the flow and make the best out of all of it. If your spouse has that going for them? You guys will be fine.
Oh, I almost forgot: travel light and keep your crap out of his way.