A few years ago before 'garages' were the norm, people were stuffing Jeeps/rockcrawlers/sand rails in them. Or maybe they were just ripping out the garage wall... I read where one guy had a Miata in there (only slightly larger than a Smart). The limiting factor in most of the TH's now is the garage length, not necessarily the carrying capacity. Because of the popularity of the SxS, the TH builders have adapted to building only up to their size, and leaving the rest of the trailer for a HUGE living space.
If I remember correctly, Forest River Work & Play is one that you may be interested in.