Most new RV trailers today have a enclosed under belly making the main frame rails almost impossible to see. Even if the main frame rails can be seen most folks wouldn't know the foot per pound weight just by looking.
RV trailers sidewalls figure into the load carrying ability unlike a flatdeck or equipment trailer.
Many of the equipment trailers websites can have the I beam foot per lb weights for their advertized gvw trailers.
You also might do a 1-800 or a email to some of the toy hauler mfg for info on the I beam size in the gvw your shooting for.
I haven't looked but NATM or RVIA webs may have builders specs.