As others mentioned, easy to eat snacks and drinks, plus some entertainment (if not movies because of motion sickness maybe music they can listen to and sing along to?) is key to getting through those long drives.
We did a 19 hr drive back in 2014 with 3 little ones (just-turned-6, 3 1/2, and just-turned-1 year old), and we were pretty hesitant about it but it made more sense for us to drive than to fly at the time. We left in the late afternoon, stopped to eat dinner two hours into our trip and drove straight the rest of the night only stopping to refuel and bathroom breaks. Our kids slept the whole way, only waking up once to go potty, and then going right back to sleep until the next morning when we stopped for breakfast. They went right back to sleep after that for a while and kept themselves entertained the rest of the trip watching movies and playing with their leapfrogs/kindles. Also, for the stretches they were awake for, we made a point to stop a couple of times to let them stretch their legs and run for a bit. Nice little breaks for everyone. In the end, IDK if it was because we got through the longest stretch of the drive at night while they slept that made the drive so much easier but it worked out really well for us and with zero meltdowns.