Potty stops are the hardest thing during the summer when you drive long distances alone. Obviously, you can't leave your dog in the car during the summer. Rest stops often have "no pets allowed inside" rules. That makes potty stops for the driver a little complicated. However, I found a solution:
I stop at Tractor Supply, Petco & Petsmart stores along the way -- and plan these stops out at 2-3 hour intervals. Petco and PS are in bigger towns; Tractor Supplies are sprinkled through rural areas. They won't mind you going inside to use that facilities with your dog. If you happen to be a woman traveling alone, it's also a safer place to stop than a lonely rest stop on the highway (and cleaner than a gas station restroom). They'll also have somewhere outside the dog can go. My dog loves these stops. (BONUS: there's often a Starbucks with a drivethrough nearby a Petsmart...not sure why, but it's definitely a pattern.)
I give water when we stop. Mine nearly always goes off his food on multi-day trips, but I do offer food on his regular schedule.
On multiday trips, I try to find a dog park to visit just after the sun comes up -- there's never anyone there that early, the dog can stretch his legs and fetch a ball, and take an off-leash poop before we get on the road.
The only antinausea drug I know of for car sickness for dogs is Cerenia. It's about $30/pill from the vet. It would be better to acclimate the dog to the car with lots of short trips, with increasing duration, for a few months before the trip.
For multiday trips, I plan out restaurants that are dog friendly so that I can get a healthy meal that isn't take-out, and my dog can chill on the patio. I also have my dog registered as an approved "camper" in the "Camp Bow Wow" chain. They have both cage-free and kenneled options. Getting in with them requires passing a day-long interview/trial so that they know the dog isn't a butthead, and being up-to-date vaccinations (so no "titering" instead). However, there's a CBW in nearly every major city, and they're all nearly identical (franchises), so when I want to go shop, sightsee, visit someone who doesn't want to see my dog, or catch a movie, I can park him at CBW for a half day of boarding (for about $20). They'll also wash him for me if we've had outdoor adventures leaving him funky smelling. It's made traveling SO much easier.
This kind of travel really reveals the joy of having an even-keeled, go-anywhere, decently trained, non-reactive dog. I seriously loved the four-day road trip I took with him recently.