Unloading does not give horses a rest. Not sure where this is coming from. Way less stress to leave them in the trailer than to unload and load in some strange area next to a highway.
Leave the horses in the trailer, tie the buckets, maybe those soft rubber, black recycled ones, in the trailer ahead of time, then figure out how to get a hose in the window, surely you have some windows or something. Lots of small battery operated pumps. Or even a hose and a funnel, and several 2 litter soda bottles of water.
For an 11 hour drive, figure on a pit stop every three, leave the horses loaded, just park in the shade, open up some ventilation and give them some wet hay and water. And let them chill for a half hour.