I've done it straight through for 15 hours in my two horse BP a few times. My horses like to travel so, as long as they have hay to munch on, they are happy as clams til they smell manure instead of gas (at the gas station stops) and know it's time to get off. I do, when I stop for gas, let them rest at the standstill for a good 15-20 minutes or so.
That said- I normally prefer an 8 hour driving day tops, particularly since I am almost always driving alone and it becomes a driver safety/fatigue issue. And no matter what my 'plan' is, I take what the day gives me- if there is lousy weather needing slower going, or a delay due to construction/accidents, I don't push to make a certain amount of mileage.
Unless you are commercial I think you can go straight through. If you are commercial, you have to adhere to the DOT regulations and take a certain number of hours rest every 10 hours I think. I rode with a husband and wife commercial team from FL to OK last October in their 12 horse semi and they took turns driving.
Every man has a right to his opinion, but no man has a right to be wrong in his facts. Bernard M. Baruch
We have traveled that several times. We always leave our horses in the trailer when we stop( fuel, br) as just the break from balancing seems to be enough, offer water etc. Getting them straight from A to B asap is easier on all of us.
I usually haul straight through, but we stop to rest to get fuel, bathroom breaks, and to eat. So they usually get a 15 minute break every 3-4 hours. The only time when I unloaded them is when we were going through Missouri and it was 100 degrees. We unloaded and hosed them off every 2 hours.
I have travelled about 14 hours no rest. Did it in our 2 horse bumper pull, and had them taken in a commercial large trailer with a stall and a half untied.
My horse was MUCH less tired coming off the ride in the commercial trailer (which would sway and move around much less than the BP trailer). We were going to a show and the year with the BP trailer ride he laid down and slept constantly (even slept deep enough to dream like a dog) at the show, though performed well. THe commercial trailer he only laid down a few times, like he would at any show with a short trailer ride.
If you have a gooseneck that would be better than a bp however, my horse was fine and even after having to trailer home 7 days later he was happy and healthy after both trips but definitely more tired with BP.
I would also think it depends on the type of driving you're doing. For example if your trip takes you through or nearby lots of cities, you are going to have a lot of stops and turns, opposed to mostly highway driving in non-congested areas, where you can basically cruise. I'm sure your horses can tolerate the straight highway driving for much longer than they can tolerate frequent detours through/around cities.