I had a good friend die in his sleep the night of his bachelor party from using an LP lantern to heat the inside of his camper shell, he thought if he cracked a window that he would be safe. He got really drunk and decided to escape to his camper shell, it was a horrible deal that I will never forget as long as I live.
Don't overthink it, buy the $25 CO sensor if you are going to risk it. By the time you feel the effects CO poisoning, it's often too late.
As you can see, there is good reason for me to harp on people about the dangers of CO poisoning.