In Illinois, you'll want to get it licensed. Otherwise you'll be at the mercy of any state police officer who happens to see it.
Here's what it breaks down to. If the trailer body was made by a company, and has a identification number, its not that bad. If it does not have a manufacturers ID, then it classifies as a homebuilt trailer. You'll have to submit pictures, and possibly have it inspected by the sec. of state police. Go here: http://www.cyberdriveillinois.com/pu...toristpub.html
and scroll down until you see the "Affirmation for Homemade Trailer" and "Homemade Trailer Statement of Construction".
Keep the total weight under 5000 lbs to make it easier on yourself too. You'll need lights, license plate holder, and safety chains as well. You should be able to get the trailer inspected first and then build on top of it, which would be easier to pass than having it inspected with your smoker on top of it.