If running main engine, may be a manifold exhaust or baffle leak being blown back into the cab. Can you smell it while driving? Crack window, get a CO detector ($30 at Walmart, you should have one anyway), get to repair shop.
Like other said it probably anti gel. That stuff smells bad after you open a bottle. Even if cap back on.
And a day goes by with the wife and husband, and all the animals in that little truck. 1 post so far by the OP.
I don’t keep anything in the truck for that reason...... 2 side boxes the fuel gloves and anti gel and oil all go there......
I had a Volvo that would do that..... Ended up being the flex pipe under the cab going to the def. When the truck would do a rolling regen the back pressure..
Asking for help and not giving an update - that; at least we are still alive, and found/solved our problem, is becoming the norm on here. Many of you gave good ideas what it could possibly be causing the vapor issue in the cab. Follow up? Are you still alive??? HEELLOOOOOO?????