I realize the OP is in Illinois. Since you are in Iowa, I think these are the Iowa rules:

http://www.iowadot.gov/mvd/omve/truckguide.pdf  It is a PDF on page Sec. 1:61

A semi-trailer exclusively used in an agricultural operation may be considered an implement of husbandry if reconstructed by permanent attachment of a converter dolly. Reconstructed semi-trailers may be pulled by farm tractors and must meet the lighting requirements for implements, may claim exception from axle weight limits, and do not need to be registered. Semi-trailers connected to a detachable converter dolly are not reconstructed and not privileged to the implement exceptions.


There are other rules that apply but a friend claims this rule exempts him from axle weight and bridge laws.  The key is non-detachable converter dolly. 

He uses an air compressor attached to his PTO to power the air brakes with a hand valve inside the cab. 

Reconstructed Semi-trailers