No, that's absolutely not the case. You can carry your truck gun with nearby loaded magazines, the loaded magazines and unloaded rifle can be in the same container, you just can't have a loaded magazine inserted into the rifle while driving without running afoul of MCL 750.227d. That law, reprinted below, requires firearms other than pistols to always be transported unloaded
and in a state not readily available to occupants of the vehicle (either cased, taken down, in the trunk, or in some other space not accessible to the occupants of the vehicle). The Michigan State Police published a
Legal Update bulletin to Michigan LEOs, and it specifically reminds officers that loaded magazines may legally be transported in the same case as unloaded rifles and shotguns.
(1) Except as otherwise permitted by law, a person shall not transport or possess in or upon a motor vehicle or
any self-propelled vehicle designed for land travel either of the following:
(a) A firearm, other than a pistol, unless the firearm is unloaded and is 1 or more of the following: (i) Taken down. (ii) Enclosed in a case. (iii) Carried in the trunk of the vehicle. (iv) Inaccessible from the interior of the vehicle. (b) A pneumatic gun that expels a metallic BB or metallic pellet greater than .177 caliber unless the pneumatic gun is
unloaded and is 1 or more of the following:
(i) Taken down. (ii) Enclosed in a case. (iii) Carried in the trunk of the vehicle. (iv) Inaccessible from the interior of the vehicle. (2) A person who violates this section is guilty of a misdemeanor punishable by imprisonment for not more than 90 days or
a fine of not more than $100.00, or both
Interesting fact:
MCL 750.227c outlaws the transport of loaded firearms other than pistols inside vehicles in general (sailboats, aircraft, motorboats, or any "vehicle propelled by mechanical means"), not just land vehicles like MCL 750.227d
TL/DR: The way you describe transport of your AK "truck gun" is 100% legal in Michigan. Enjoy your stay, the northern parts of Michigan are wonderful.