To clarify, looks like you are safe to operate your RD in Illinois.
Since you are not using the vehicle for commerce, you are not a commercial vehicle.
(6) Commercial Motor Vehicle.
(A) "Commercial motor vehicle" or "CMV" means a
motor vehicle used in commerce, except those referred to in subdivision (B), designed to transport passengers or property if:
(i) the vehicle has a GVWR of 26,001 pounds or
more or such a lesser GVWR as subsequently determined by federal regulations or the Secretary of State; or any combination of vehicles with a GCWR of 26,001 pounds or more, provided the GVWR of any vehicle or vehicles being towed is 10,001 pounds or more; or
(ii) the vehicle is designed to transport 16 or
more persons; or
(iii) the vehicle is transporting hazardous
materials and is required to be placarded in accordance with 49 C.F.R. Part 172, subpart F.
(B) Pursuant to the interpretation of the Commercial
Motor Vehicle Safety Act of 1986 by the Federal Highway Administration, the definition of "commercial motor vehicle" does not include:
(i) recreational vehicles, when operated
primarily for personal use;
(ii) vehicles owned by or operated under the
direction of the United States Department of Defense or the United States Coast Guard only when operated by non‑civilian personnel. This includes any operator on active military duty; members of the Reserves; National Guard; personnel on part‑time training; and National Guard military technicians (civilians who are required to wear military uniforms and are subject to the Code of Military Justice); or
(iii) firefighting and other emergency equipment
(including, without limitation, equipment owned or operated by a HazMat or technical rescue team authorized by a county board under Section 5‑1127 of the Counties Code), with audible and visual signals, owned or operated by or for a governmental entity, which is necessary to the preservation of life or property or the execution of emergency governmental functions which are normally not subject to general traffic rules and regulations.
Also, if you get the cop to admit that he pulled you over because he thought you had a radar detector you can probably get out of the ticket easily.
Disclaimer: This is not legal advice!
(625 ILCS 5/12‑714)
Sec. 12‑714. Possession and use of radar detection devices prohibited.
(a) No person shall operate or be in actual physical control of a commercial motor vehicle as defined in Section 6‑500(6) of this Code while the motor vehicle is equipped with any instrument designed to detect the presence of police radar for the purpose of monitoring vehicular speed.
(b) Notwithstanding subsection (a) of this Section, a person operating a commercial motor vehicle as defined in Section 6‑500(6) of this Code, who possesses within the vehicle a radar detecting device that is contained in a locked opaque box or similar container, or that is not in the passenger compartment of the vehicle, and that is not in operation, shall not be in violation of subsection (a) of this Section.
Any person found guilty of violating this Section shall be guilty of a petty offense. A minimum fine of $50 shall be imposed for a first offense and a minimum fine of $100 for a second or subsequent offense.
(c) The radar detection device or mechanism shall be seized by the law enforcement officer at the time of the violation if the offender has previously been convicted of violating this Section. This Section shall not be construed to authorize the permanent forfeiture to the State of any radar detection device or mechanism. Any such device or mechanism shall be taken and held for the period when needed as evidence. When no longer needed for evidence, the defendant may petition the court for the return of the device or mechanism; provided the defendant shall prove to the court by a preponderance of the evidence that the device or mechanism will be used only for a legitimate and lawful purpose.
(d) No commercial motor vehicle, or driver of such vehicle, shall be stopped or searched by any law enforcement officer solely on the basis of a violation or suspected violation of this Section.
(Source: P.A. 90‑89, eff. 1‑1‑98.)