BDM, congrats on the horse trailer coop! I've been using a 2-horse '73 Miley horse trailer for an intermediate brooder for the past 3 hatching seasons, this will be year 4 that it goes into service. I took the center divider out so the entire floor area and the manger area is usable floor space for the birds. I have hardware cloth over all the door openings so I can open it all up during the day. It gets wickedly hot here each summer tho and the trailer is dark brown so I park it in the shade, and I also have a big box fan that gets ziptied onto the wire to keep the air moving when the summer temps really kick in. When I need to clean it out between batches of keets I just unplug my power cords, hook my truck to it, haul it over to my burn pile and just sweep it out (LOVE that part!). I also love that it's reliably predator and rodent proof. I added some roosts at different levels and the keets (and poults) are usually up in the manger area (that I bed with straw) as soon as they discover that they can hop from the roosts over to the mangers (wire over the openings, and I close the doors at night). I use the tack compartment under the mangers for feed and brooding supply storage, but I did have an old ancient metal chick brooder in there for my newly hatched keets my first season (can't remember the brand but it was a good one, back in it's day). I did not trust that ancient brooder contraption thing tho, or that the baby keets would get enough airflow at night with the door closed (plus it was hard to watch them or reach in and catch them, so that only lasted a little while... lol.
Congrats again on your mobile coop score, can't wait to see it once you get it all decked out and filled with birds.