Fortunately, I have never gotten involved with vehicle design in a serious way. It seems it would be a mess, especially starting from scratch.
For a normal over-the-road truck, the actual load carried should be no problem, as it's limited by legal loading. So assume it's a big chunk of iron in the middle of the trailer, and there's your load.
The problem arises in that at some point, you have to figure the maximum dynamic loading. IE, figure the biggest chug hole you'd ever hit, or hopping a curb at speed, or series of dips to match the natural frequency of the trailer or whatever you can come up with. Maybe this is specified and standardized somewhere; if so, that SAE site would be the place to start looking. Or it could be proprietary information, gleaned from years of experience at different vehicle manufacturers. Proper roll-over stability would be another issue.
I know you can just buy off-the-shelf wheel-and-axle sets for semi trailers. Perhaps you could come up with the maximum load they can handle, and work backwards from that.