A lot of places require a class A to tow over ~15k. California has a RV exemption for 5th wheel trailers between 10k and 15k, otherwise all trailers over 10k in California require a class A to tow. And yes, I have been pulled over by an over zealous CHP on the grapevine to check my license and trailer reg. After seeing it, his comment was "nice trailer". I guess 40' trailers can draw attention.
An 18k gvwr trailer pulled by a truck with an 8001 lb or higher gvwr would seem to put you into a class A.
page 15
"Chauffeur’s Licenses
An Indiana chauffeur’s license grants the holder all the privileges of a driver’s license. In addition, the holder of a chauffeur’s license is permitted to operate vehicles with a gross vehicle weight rating of at least 16,000 pounds, but not more than 26,000 pounds (whether single vehicle or combined gross vehicle weight) when used to transport property for hire.
To operate a vehicle with a gross vehicle weight rating of 26,001 pounds or more you must have a commercial driver’s license."