Well, it is a little cooler in Kentucky than in Texas.
It is starting to cool off here in Texas, because the highs are less than 100 degrees now. Today here it is 99 degrees for the high and 76 degrees for the low. The 10-day forecast does not show the temperature going below 70 degrees.
The...
If chickens were locked in a metal horse trailer during the day here in Texas, I think the chickens would die from the heat.
I know it is cooler up north, but I am guessing you let your chickens out of the coop during the day, right?
If you parked the trailer under a shaded area, it might work. You will need to put wire over the openings to keep predators out and have a way to close them off when it is cold. You still need a little ventilation in the winter, but you don't want air blowing in on your chickens.
Do you plan...
The problem I see with using a horse trailer for a coop is that the metal would get very hot in the summer and very cold in the winter.
I would suggest getting some free wood pallets and building a chicken coop.