Hi B.,
Lots of good advice. We did the night thing when kids were little; sometimes we still do the "very early morning" start with our teens, they will sleep for awhile just like toddlers!! lol
First off, plan for it to take more than 8 hours. Try not to schedule anything for right after you arrive. You'll stress about getting there on time, and what with possible delays along the way (accidents, construction etc) you might not make it at the right time.
When you do stop for whatever reason, take care of everyone/everything before you get back on the road--e.g., bathroom/diaper, exercise, food/snacks, and gas. It is really frustrating to stop for bathroom, get back on the road, have baby fall asleep, then have to stop for gas half an hour later and have baby wake up and fuss!!
If you are still nursing you can actually nurse in the car if you are not driving--keep child in car seat, but you will have to go into contortions. But it works.
Since you will have other adults in the car, they can take turns watching baby while you use the rest room. But if you ever travel alone with a baby, put him/her in a stroller and use the handicap rest room or family rest room. the stroller will fit in there, and baby will be reasonably confined while you do what you need to.
Put an old towel or blanket or flannel-backed tablecloth on the car seat (not the baby carseat, the one that is part of the car--hope that made sense) to catch crumbs, spills etc.
Have a camera handy to capture anything cute baby and grandparents do while traveling, at rest stops etc. you'll make good memories.
Have fun!
K. Z.