A fixer-upper trailer tells me you need some serious look-see done by true professionals who know what to look for in rehabbing horse trailers.
As walkin mentioned...that floor support system is going to take the biggest beating, and is overlooked how many times...
Even you did not mention the undercarriage checking just putting new flooring down...
Check the wiring that all is enclosed in steel conduit and up away from the road, not being caught as you go on and off of roadways for gas, bathroom, food and in the evening a place to stable the horse. DO NOT
unload that horse anyplace but your nightly destination for the horses safety.
The horse is going to be stressed hauling long days in a very hot trailer..
You unload that animal it may not go back on and now what???
Trailering at that time of the year days coming cross-country are often 100 degree by 10AM...
I am hoping your plan is to start driving around 11:00 PM as the heat of day leaves, drive all night long and be done and off the road ASAP so the animal can have some cool-down time of a good hosing and quiet time in the shade of a non-moving stall or paddock.
You mention water and carrying enough of it...
Invest in a inline filter for a garden hose and start using it now
...filter all her drinking water so no matter where you are the water tastes the same to her.
Otherwise, buy jugs of some name brand bottled water so she will drink, flavor the water with whatever you can starting now so again, it is something very familiar to the horse.
How long are you figuring this trip shall take?
Figure a bale a day cause when traveling I would be offering hay all the time, water often while driving and if the animal wastes, they waste...then you need 3 bales at destination for a slow and gradual changeover to new hay.
I would also be very hesitant to grain my horse on such a trip... again, what ever you are going to feed/ do currently feed make sure when you arrive at that destination that brand is available so you can start the introduction again while she is eating her same hay product.
Vet care...make sure the horse is fully vaccinated not only for what your area of the country has but for your destination has...it is different often.
The horse needs those vaccinations done now so the body has time to build the appropriate antibodies so best protection is assured for the traveling, stressed animal.
You may need special paperwork to cross state to state as some states have significant requirements different...do that now, not wait.
Its nearly July and you have several long holiday weekends that no paperwork will be done during, all of which needs done in advance and takes time.
Make sure your truck and trailer meet each states vehicle code for traveling.. Drivers have correct licensing, trailer meets all vehicle code and you have proper insurance covering your travel interstate needing done.
Laws have changed, and regulations are tightening significantly even for private owned horse trailers. It is your responsibility to be in compliance of those regulations.
And near finally....a HUGE one...
You will be traveling major interstates at highway speeds far exceeding what driving you do on local roads.
Your horse needs to be comfortable traveling on those speed sanctioned roads. Road vibration is huge in a regular horse trailer...leg wearying and exhausting. Make sure that animal is able to rest in shade in quiet roadside areas not just sitting cooking in a trailer at a truck stop...there is a difference in allowing for a rest period.
And here is the other HUGE one...
When you replace those trailer tires...
Because you will be traveling interstate highways at high speed... Make sure your tires are rated for enough weight carried and high enough speeds to allow safe traveling.
Trailer tires are commonly only 65MPH rated, you need higher ratings because you will be traveling at the max speed you can within the law of that state... Do not
exceed the speed rating of your tires, period.
Do check daily those tires for problems, check air pressures daily and that you not have leaking bearings, and tire tread is evenly wearing. You should be doing a minor inspection of that trailer at every rest period, fueling stop and every-time you stop for a extended few minutes to catch a problem before it becomes a major problem.
When you purchase those new tires, you need to purchase 2 extra spare tires, have at least 1 mounted on a good rim and all tires need to be speed balanced for best ride of the horse in that trailer.
Attention to details now, ahead of travel will make your long distance trip less stressful for you and the horse.
Don't cheap out on maintenance on either vehicle cause once away from home, you know nothing about the area you may breakdown in or have a issue near..you are at the mercy of your location.
Truck fully inspected, serviced and reliable. check!
Trailer, fully serviced, newly maintained and all has been checked... check!
Every weld, check that hitch for road-worthy safe, every window, every door, every light inside and out works, brakes new and properly adjusted, wheel bearings new and a extra set already greased carried on the trip for just in case... check!
Paperwork in order, vaccination records, coggins, health certificates that meet DOT federal and state guidelines of states you cross, insurance on vehicles, towing insurance/roadside assistance not only for the truck but a horse trailer occupied, .... check!
Those things should get you headed in the right direction...
Today, make sure your license is going to be adequate as laws are changing I can't stress enough.
Many are driving illegal and not know it in their rigs...the onus is on you to be legal in all states you travel not just where you are from. check!!
You are going to be a target with out of state plates as you travel, make sure every "T" is crossed and "I" is dotted before you ever pull out of the driveway on your start of travel = least stress on you = less stress on the animal you haul.
Safe travels. double and triple checked!!!