They make water storage bags for camping and emergency preparedness from individual human-size servings up to 100 gallon bathtub size bags. Something in an intermediate size could be useful... search "water storage bags" on amazon
/walmart/bass pro shop/anywhere that sells outdoor gear.
Something else you could do - we're adding interior lighting as we fix up our old trailer.
I can't think of what exactly they're called. We got them at Walmart.
It's a single fixture from the auto section that had 2 small flood lights that would swivel independently from each other about 180*.
We're going to mount it upside down to the ceiling, right inside the divider bar in the back so we can point 1 light into the trailer and 1 light outside to the back.
It will be connected to its own button so if we're having to load in the dark, the horses will be able to see.
I'm also going to add padding to the inside walls and to the divider so the horses don't lean against bare metal. Most likely I'll rewrap the butt chains to match since the rubber on them is getting dry-rotted.
I'm planning on getting outdoor upholstery-type fabric so it's easy to clean, durable, and (hopefully) a cute pattern.