How much water should be in my horse trailer?

You should stop and water your horse every three to four hours for about 10-20 minutes and if its safe take her out to stretch her legs. Even still stopping will allow the horse to take a break and unbrace their legs A horse drinks about 10-20 gallons per day and if its hot outside than they will drink more.

If you are only doing a one day trip then that would be more then enough for your horse. Basically you want to average about how much she drinks a day and then bring about 3 gallons extra just in case. However I would advise turning this into a two or three day trip as that is a awful long time to have a horse in a trailer. When we travel with ours I don't do more then 8 hours stretches as it can be very tiring for them, 17 hours that's an exceeding long day!

That should be enough water. You could also go on the side of caution and buy a third jug. Also, have a back up plan and buy some apple juice, molasses, or something else that she will like to flavor the water, so if you do need to use unfamiliar water, you can hide the new taste so she still willl drink

Damp hay is a great idea, no dust and less water, just don't let it mold if it is hot. A horse will drink a 5 gal bucket when stalled at night when on hay.

You should start dumping your water out of those plastic containers well before you leave, as I would think they would give the water an off taste and smell.

I have heard jello is a good thing to put in water. also you might have a little bottle of Vicks or menthol to put a dab in his nose before you give him water.

Also most towns would have city or county arena with lots of corrals. You might be able to pull into one of those to give your horse a turnout and you a little rest. I know I have slept in my truck many nights on a long haul to give us both a rest while at a local rodeo grounds.