[dsraven]

CO,
odorless, colorless, tasteless.
co gets into your lungs when you breathe, enters the blood stream when your blood goes through the lungs to get re-oxygenated and bonds with your hemogloben, taking up space where the oxygen would normally be bonded. levels of oxygen are then reduced. headaches, nausea and vomitting are symtoms. it takes a long time for the co to leave your system and if the dose is enough you could be put in a hyperbaric chamber to assist with that. the level of ppm you are able to be in is also related to your age and history. if, say, you have a small level per day then over time it builds up because your system can't get rid of it as fast as you are getting it added. you could be feeling lethargic and think you just had a bad week when actually it is the co talking. you're tired because you're not really charged up on oxygen like you should be.
some monitors will read by co or o2 levels in the area. just know that a lower than normal o2 reading means there is something else in the area displacing the o2. it would be good to know the actual levels of other toxins or gases inside the cab after a fill up with an inside the cab style fuel tank. compare that to the lower explosive level of that chemical and see where youre at. exposure to gasoline vapors has caused cancer in lab rats. why take the chance?
vehicles get their fresh air through vents or the heater, barring a bunch of holes in the cab, so a parked running vehicle can get outside air with the heater running. a good reason to have the exhaust dump out the rear, like new cars have had for years.
a good way to check for leaks is run the heater on high fan, close doors and wash the vehicle with really soapy water. bubbles will show where the big leaks are. door seals, sliding window seals, door handles, vent windows, etc etc. look under the hood as well. holes in the firewall bring in engine bay odors and contaminants.
anyway, hope you get that fixed so you can enjoy life and feel energized.