The key to high mileage days is to hit the road early and keep your fuel and food stops as short and efficient as you can.
You will be stopping more often for fuel than you might think since most tow vehicles have a fairly short range when considering the poor miles per gallon towing. Guaranteed you have days ( and days, and days ) of headwinds or quartering headwinds, and that totally kills your fuel economy, meaning more fuel stops.
You're going to need to hit the road early for sure each day.
Most people are not acclimated to rising very early day after day, and maintaining concentration for 10 to 12 hours solid, day after day, so it takes a toll, as some others have said on about the second or third day.
The first day comes fairly easy on enthusiasm. The second day high mileage day comes thru determination. The third day, a person can start getting punchy and irritable, which is unsafe.
I've done many high mile days over the years in cars, in pickups pulling trailers, and on motorcycles, but these days, with the camper in tow, I'm like a lot of folks who have already posted above, I like to ease into the day by hitting the road by 9:00 to 10:00, and stop by 3:00 to 5:00PM. That makes it to where I can set up and relax and enjoy the last hours of daylight, take a long walk with the dog, and enjoy a nice dinner. That winding down period at the end of the day for me leads to a restful sleep, which is obviously important for the next day.