Every time I've asked this question about particular MH's the answer has been that they couldn't handle the weight of something like a Smart directly over the rear axles even though the overall cargo capacity might seem to be large enough to handle it. I believe a Smart weighs ~1800 lbs so you would need at least that much extra capacity on the axles and you would still like to be able to carry a few other things. I'll be interested in reading about what you find out.
Joel (AKA docj)--
RV Technology Specialist