In TN you have to have a carry permit to have a loaded handgun even in your vehicle (being a commercial vehicle or not is irrelevant. Otherwise it is a violation of TCA 39-17-1307
39-17-1307. Unlawful carrying or possession of a weapon. —
(a) (1) A person commits an offense who carries with the intent to go armed a firearm, a knife with a blade length exceeding four inches (4²), or a club.
(2) (A) The first violation of subdivision (a)(1) is a Class C misdemeanor, and, in addition to possible imprisonment as provided by law, may be punished by a fine not to exceed five hundred dollars ($500).
(B) A second or subsequent violation of subdivision (a)(1) is a Class B misdemeanor.
(C) A violation of subdivision (a)(1) is a Class A misdemeanor if the person's carrying of a handgun occurred at a place open to the public where one (1) or more persons were present.
(b) (1) A person commits an offense who possesses a firearm, as defined in § 39-11-106, and:
(A) Has been convicted of a felony involving the use or attempted use of force, violence or a deadly weapon; or
(B) Has been convicted of a felony drug offense.
(2) An offense under subdivision (b)(1) is a Class E felony.
(c) (1) A person commits an offense who possesses a handgun and has been convicted of a felony.
(2) An offense under subdivision (c)(1) is a Class E felony.
(d) (1) A person commits an offense who possesses a deadly weapon other than a firearm with the intent to employ it during the commission of, attempt to commit, or escape from a dangerous offense as defined in § 39-17-1324.
(2) A person commits an offense who possesses any deadly weapon with the intent to employ it during the commission of, attempt to commit, or escape from any offense not defined as a dangerous offense by § 39-17-1324.
(3) A violation of this subsection (d) is a Class E felony.
(e) It is an exception to the application of this section that a person authorized to carry a handgun pursuant to § 39-17-1351 is transporting a rifle or shotgun in or on a privately-owned motor vehicle and the rifle or shotgun does not have ammunition in the chamber. However, the person does not violate this section by inserting ammunition into the chamber if the ammunition is inserted for purposes of justifiable self-defense pursuant to § 39-11-611 or § 39-11-612.
(f) (1) A person commits an offense who possesses a firearm, as defined in § 39-11-106(a), and:
(A) Has been convicted of a misdemeanor crime of domestic violence as defined in 18 U.S.C. § 921, and is still subject to the disabilities of such a conviction;
(B) Is, at the time of the possession, subject to an order of protection that fully complies with 18 U.S.C. § 922(g)(8); or
(C) Is prohibited from possessing a firearm under any other provision of state or federal law.
(2) If the person is licensed as a federal firearms dealer or a responsible party under a federal firearms license, the determination of whether such an individual possesses firearms that constitute the business inventory under the federal license shall be determined based upon the applicable federal statutes or the rules, regulations and official letters, rulings and publications of the bureau of alcohol, tobacco, firearms and explosives.
(3) For purposes of this section, a person does not possess a firearm, including, but not limited to, firearms registered under the National Firearms Act, compiled in 26 U.S.C. § 5801 et seq., if the firearm is in a safe or similar container that is securely locked and to which the respondent does not have the combination, keys or other means of normal access.
(4) A violation of subdivision (f)(1) is a Class A misdemeanor and each violation constitutes a separate offense.
(5) If a violation of subdivision (f)(1) also constitutes a violation of § 36-3-625(h) or § 39-13-113(h), the respondent may be charged and convicted under any or all such sections.
[Acts 1989, ch. 591, § 1; 1990, ch. 1029, § 6; 2007, ch. 412, § 1; 2007, ch. 594, § 3; 2008, ch. 1166, § 1; 2008, ch. 1176, § 1; 2009, ch. 431, § 1; 2009, ch. 455, § 6.]
There is a list of defenses in 39-17-1308, but being in your vehicle is not one of them
39-17-1308. Defenses to unlawful possession or carrying of a weapon. —
(a) It is a defense to the application of § 39-17-1307 if the possession or carrying was:
(1) Of an unloaded rifle, shotgun or handgun not concealed on or about the person and the ammunition for the weapon was not in the immediate vicinity of the person or weapon;
(2) By a person authorized to possess or carry a firearm pursuant to § 39-17-1315 or § 39-17-1351;
(3) At the person's:
(A) Place of residence;
(B) Place of business; or
(C) Premises;
(4) Incident to lawful hunting, trapping, fishing, camping, sport shooting or other lawful activity;
(5) By a person possessing a rifle or shotgun while engaged in the lawful protection of livestock from predatory animals;
(6) By a Tennessee valley authority officer who holds a valid commission from the commissioner of safety pursuant to this part while the officer is in the performance of the officer's official duties;
(7) By a state, county or municipal judge or any federal judge or any federal or county magistrate;
(8) By a person possessing a club or baton who holds a valid state security guard/officer registration card as a private security guard/officer, issued by the commissioner, and who also has certification that the officer has had training in the use of club or baton that is valid and issued by a person certified to give training in the use of clubs or batons;
(9) By any person possessing a club or baton who holds a certificate that the person has had training in the use of a club or baton for self-defense that is valid and issued by a certified person authorized to give training in the use of clubs or batons, and is not prohibited from purchasing a firearm under any local, state or federal laws; or
(10) By any out-of-state, full-time, commissioned law enforcement officer who holds a valid commission card from the appropriate out-of-state law enforcement agency and a photo identification; provided, that if no valid commission card and photo identification are retained, then it shall be unlawful for that officer to carry firearms in this state and the provisions of this section shall not apply. The defense provided by this subdivision (a)(10) shall only be applicable if the state where the out-of-state officer is employed has entered into a reciprocity agreement with this state that allows a full-time, commissioned law enforcement officer in Tennessee to lawfully carry or possess a weapon in the other state.
(b) The defenses described in this section are not available to persons described in § 39-17-1307(b)(1).
[Acts 1989, ch. 591, § 1; 1990, ch. 1029, § 7; 1993, ch. 200, § 1; 1996, ch. 1009, §§ 20, 21; 1997, ch. 476, § 3; 1999, ch. 295, § 1; 2003, ch. 144, § 2.]
Now if the gun was in his sleeper...he might could make the argument that it was in his residence. Let us know how it turns out.