We have traveled from Illinois to Tennessee (9 hours) and West Virginia ( 12 hours) a couple times each year with our kids since about that age on. Now they are 2 and 4, and I'll tell you, it is much easier when they are that little, they don't excpect to be able to run and move around too much, cause they can't yet! much easier to deal with diapers than a newly potty trained or training child!!!
I nursed still at that point, but once we were on to bottles, we just took a large thermos of VERYHOT water and then a regular water bottle of cool water, to mix and get it to "just right" temp. I would encourage you, if you are formula feeding, though, to start to train your baby to drink bottles at room temperature, maybe while you are visiting, start gradually just make the bottles cooler and cooler, and eventually just stop at room temp, SOOOOO much easier overall for mixing bottles at home and away from home if you don't have to worry about warming it, lots of my friends did this, I nursed till 9 months, so mine wanted it body temp, once they finally took bottles - how I wished they would take it room temp!
I would feed right before we left, so they would take a nice long nap on a full tummy in the car. We would stop, usually at a rest stop every 2-3 hours and change diapers, nurse or give a bottle, and lay them down on a blanket outside to stretch and roll around a little, get some fresh air. We didn't feed while driving, harder to burp them, unsafe to take them out of the seat while moving and if they spit up, OH YUCK!!! If we ran out of hot water, we would stop at a restaurant and ask for some, or at a gas station and just microwave some. Bring diapers, bring any "sleep music" you may use in their bedrooms at home, to play in the car, new toys, or some they haven't seen in a while. Read out loud to them (doesn't have to be baby books, you could ead your latest novel, just hearing your voice and the words is good for them)! Sing silly songs and do finger plays ( Itsy Bitsy Spider, Where is Thumbkin) just plain talk to them about what is outside the car, where you are going, etc. That baby will probably be LOVING all the undivided attention, because even though there are no siblings, at home, there is always housework, the phone, tv, computer....this way it is you and him! Take turns in the back seat if you want to take a break, let your SO do baby duty for a while too.
I don't think it will be nearly as bad as you might think, and remember, if he is upset, you can ALWAYS stop and take a break, you don't HAVE to push through in a big hurry, TAKE TIME and make it about the trip, and not just the destination! We like to stop and eat at Cracker Barrel, just to browse through the store, rock in the rocking chairs and take a break, and if there is something else interesting along the way, we stop and see it!
Good luck, I think you will be fine!