DIY water tank
What are some ideas about accomplishing this? She doesn't want to spend a lot of money and doesn't care about details just wants it done so they can take showers after shows.
There are at least a hundred ways to skin a cat. But here's what I would probably do:
First, get a suitable water tank, preferably with baffles to minimize the water sloshing around inside the tank when on the road. Secure that tank to the bed so it won't come loose and make a mess if she bumps the truck into something. Depending on how many folks need how long a shower, and now thirsty are the horses, then probably only 10 to 20 gallons is all they might need.
Water is heavy, and even with a baffled tank it will slosh around, so don't be temped to install more than about a 40-gallon tank.
My RV fresh-water tank is 20 gallons. So that might work as the tank. But if the RV tanks take up too much room on the floor of the bed, then find a taller tank with a smaller footprint. And other RV trailers and LQ horse trailers have bigger fresh-water tanks if she needs more than 20 gallons.
Here's a 35-gallon tank that might work for her:
Norwesco Horizontal Leg Tanks
And here are 20 and 30 gallon alternatives:
Applicator Saddle Tanks
I don't know if those tanks are baffled, so check them out before spending your money.
Next after the tank, I'd probably use Chuck's idea of an RV water pump. 12-volt so it can be battery powered. Hook it up to either the truck batteries, or maybe have a seperate 12-volt battery just for the water pump.
(Keep a good automatic 12-volt battery charger handy, and any time you have electric power available, charge up the battery. Or if you're not around electric power, then use the generator to power the battery charger. Of if the generator is a Honda i2000, then buy the optional 12-volt connector and charge the battery staight from the i2000 12-volt outlet.)
ShurFlow is a good reliable brand for the pump. CampingWorld is one source.
SHURflo Fresh Water Pump - Item - Camping World
Here's another one:
Shurflo Pumps - The Classic 2088 12V on Sale
Note the accumulator tank (small pressure tank) available as an accessory. Yes, you'll probably want that too.
Water Pump Accumulator Tank
And here's some install instructions from the Shurflo website on how to set it up.
http://www.shurflo.com/pdf/rv/911_trouble_shooting/new/911-352-I.pdf
Now, modify that setup to have a water hose quick-connect right after the accumulator tank, and ignore all the fixtures in the instructions. Have the shower hose that includes a shutoff valve on or near the shower head on a hose that will reach from the tank to the shower area, with the quick-connect to plug it into the accumulator tank.
That will provide water "on demand" for a cold shower or to run water into something to water the horses. The pump will run only when the shower is running.
If you want warm water, then you'll have to somehow heat the water before you pour it into the tank.

Or hooking up an RV water heater is a whole 'nother project. :wink2:
Addendum. Here is a bigger/better/faster Shurflo pump for a little more money:
SHURflo On-Demand Diaphragm Pump — 3 GPM, 12 Volt, 1/2in., Model# 2088-343-135 | 12 Volt Pumps | Northern Tool + Equipment
Using a 30-gallon or 55-gallon barrel for the tank is not a good idea - they are relatively tall and skinny and have a high center of gravity, so much more likely to (try to) turn over on curves or sharp bends in the road or when turning a corner. But you might be able to "made do" with a barrel until you save a few more bucks to do it right. If you start with a barrel, be sure you figure out a way to firmly anchor it to the bed of the truck so it won't move around or turn over. Maybe if the truck bed has a heavy-duty headache rack, then you could use a motorcycle tiedown strap and tighten the tank up really tight to the headache rack? As well as tightly tie it to a tiedown near the bottom of the bed.
You can buy new water barrels from the same source as the water tanks in my previous post. I would not consider using a used barrel for my drinking water.
Maybe mount the pump and the accumulator tank on a thick piece of plywood, then figure out a way to mount that plywood on top of the bigger water tank? And the power cord for the 12-volt pump could have one of those plugs that plug into a cigar lighter or 12-volt power point?
If the horse trailer has a front tach or feed room, I'd probably prefer to mount the water tank and pump and battery and battery charger in the trailer instead of in the bed of the pickup.
If the trailer has a 6-pin horse-trailer or a 7-pin RV plug, then one of those pins is a 12-volt hot wire, which in effect is the battery charger wire. So it's a simple project to connect that 12-volt wire to the trailer battery so the battery will be charging any time the truck is running.
Lots of experience convinced me that even though a cheap 12-volt battery might work, an expensive marine "deep discharge" battery is much more reliable and long-lived. So the battery in my RV trailer is an Optima BlueTop, available at Sam's Club.