We met a mother-daughter on an organized ride who had a 3 horse slant load bumper pull trailer with a front tack room. They had 2 horses and set the 3rd stall up to hold the tack, feed and other horse stuff. Then they had a welder build some bunk bed frames in the front tack part. It was really cool, the beds were on hinges and chains that could be folded up against the wall, freeing space for dressing, etc. That way if they wanted to sleep in it they could but it didn't use up too much space. It was pretty cool.
I went out and took a couple of quick photos of ours. Sorry, Shiloh wouldn't move her butt
It was a custom order Sundowner that we bought used. Its really easy to pull and isn't humongous. There is no room for saddles is the only problem. We have to carry all the horse stuff in the back of the truck (it has a topper). Just to give you some ideas of what can be done to keep it on the smallish side....Yes I do have to sleep on that top bunk