Not a cargo trailer, but a horse trailer with living quarters would do what you are asking. You might be able to find a nice used 2 horse bumper pull, not the long 45' trailers, but a short 2 horse. I have seen a couple with a pop out on the side for the bed, most amenities, and hauls 2 horses, this can be used for toys, etc. You would probably pay more than a comparable size RV trailer, but it is built stronger, solid roof, etc. It would also have all the plumbing and holding tanks, which I would think would be the hardest part of any build.