Firearm needs to be unloaded and encased in WI. It does not need to be "out of reach." The strictest lettering of the law means that even an encased firearm is conceal carry since you know it is there and no one can see it (concealed). You can have ammunition in the magazines, but not in the firearm, but it can reside in the same box/bag. If the magazine has no rounds in it, then it can be placed in the firearm.
As for IL laws, you must keep it unloaded, encased, and locked (trunk or behind the back seat). If you get pulled over, do not consent to any searches or seizures. I was once pulled over in IL and they threatened to put me in jail for not having a FOID card. Well, that is impossible for me to get since they do not give them to WI residents. And that is a form of registration BTW. Since you are transporting the firearm, you are covered under federal law.