You would think so but I've looked and haven't seen anything. If I was going to buy an RV why not a combination so you wouldn't have to pull a trailer and could actual pull a boat or small vehicle if you wanted. Just seems more efficent to me. I would think you would need very sloped railed ramps though. Would a ceiling be high enough or the floor frame strong enough if you converted an RV though?
Fibro got me again tonight I think :) so I'm up in the middle of it.
I found some links that are interesting.
http://www.gregsrv.com/horsetrailer.html
http://store.cowboycadillac.com/site/index.cfm
They might do a custom rv horse trailer