If you have more drivers than vehicles it is always nice to switch drivers every few hours. Hubby is a relaxed driver & loves driving so our non-camping road trips are always a marathon. When we drive to our condo in Florida (from north of Toronto) we do the 20hrs of drive time in a day and a half. We like to start early (4-5am) and stop around 9pm on the first day, or later if we are not near any decent accomodations. We do as hubby calls them 'all-in-one-stops', that is gas-eat-pee, now that we dont have children travelling with us.
I pack a small(ish) cooler with cans of pop/soda with caffeine, a couple of frozen water bottles and some snacks. I try to avoid sugary snacks as they will cause a blood sugar spike followed by a crash & sleepiness. As someone mentioned vegies, with or without dip, are good, as is cheese & slices of Kolbassa with crackers and some nuts. If it's a multi-day road trip I pack some freezer ziploc bags to fill with ice for the cooler for subsequent days.
A few years ago we were in Florida and our son phoned to ask if we could drive home to his place(near Toronto), pick-up his truck & work trailer and then drive it to Calgary (a 3350 mile trip) ASAP! We left FL early the next morning, stopped at his house to get his truck, then at our house to drop off our car, unpack & repack and hit the road again. Talk about a numb bum! Thank goodness we didnt have to drive home again -his company flew us home.