I'd feel confident in saying the law does not say the vehicle is to be measured while loaded to maximum capacity.
So measure your vehicle as it sits. They are saying a vehicles GVWR. Meaning your vehicle is rated to weigh that much.....not that it does weigh that much.

This is the next place I'd try. Its your only hope.......I'm damn sure the cop that checks you is likely to say its the way best suited to get you a fine.
http://www.bmv.ohio.gov/registrar_email.html