The longest haul I did was about 8 hours. I had an old steel Cotner 2 horse straight load no dressing room. That sucker was a tank but hauled well. I used a Chevy 1500.
I have a decent amount of hauling around that 4-5 hour mark with either the Cotner or a 2 horse straight load Trail-et with dressing room.
The majority of them have been with a 1500 with no sway bars and no weight distribution hitches and mostly with 1 horse. I live in SE PA and have been to OH, WV, VA, MA, near Syracuse NY.
I have hauled with a 2500 and last year I splurged and got an F350. All of mine have the 6.5ft short bed not the 8 ft bed.
Towards the end of owning the Chevy 2500 I was thinking I might want a weight distribution hitch but then I found out the truck suspension needed work and when it had the extra weight of the trailer it had a sway.
For my Arab cross I took him to a long weekend foxhunting and trail riding. 4.5 hours each way with lots of back roads. When I got home to the barn he didn’t want to come off. He was perfectly happy staying on the trailer eating hay. I had to go up front and push him off. I figure if he wasn’t eager to come off and go in his stall for dinner then out for the night he didn’t find the trip particularly stressful.
I know a lot of people on this board don’t recommend towing with a half ton truck. Personally I don’t feel that way. I have towed for 20 years with a half ton with no issues and no I have not been towing in only flat areas. Yes I preferred my 2500 but that also had heated seats, leather and was a crew cab. The 350 is nicer but it also is leather, crew cab, diesel, heated/cooled seats, sun roof.
I don’t know anything about slant loads but I don’t see a problem with using a BP for longer trips. I think the same driving applies- plan your stops so you don’t brake quickly, be careful in the turns, easy acceleration. Don’t do anything quickly. I pay attention to that light that has been green a long time and figure it will likely be turning red soon so slow down a little bit so I have more time to stop. You see the car at the cross street ahead- plan on them pulling out ahead of you so they won’t be behind the “slow” trailer. I think that as long as the trailer is well maintained the biggest difference on how well a horse travels is if the driver can give the horse a smooth ride. Be alert and be very defensive in your driving so you have more time to stop and not jar the horse.