Has anyone had trouble breathing while in a pop up camper?

[jodo]

We have a 1989 Rockwood pop up camper, and all three of us have noticed that at one time or another, we wake in the night feeling like we can't get enough air and go outside for a while until it passes. I was the first one it happened to several years ago, and I thought it was some sort of panic attack, but it has happened to my husband twice and our son once as well, but only while in the pop up camper. Could it be that our heads are too low, or that there really is a lack of oxygen? It doesn't seem that tightly sealed, but we can't figure out what the problem is and why it has only happened a few times in the last 8 years. It has only happened to any of us while sleeping in the pop up, and never at the same time. It has always been on separate occasions. I thought maybe I was claustrophobic or something, but we wake up that way. It doesn't happen while we are in there playing cards, or watching a movie while we’re laying down at night. We have detail cleaned it, but maybe mold somewhere? I'm hesitant to use it again until we can figure out what's going on, so we rented a cabin for our vacation instead of camping this summer. I'd hate to stop camping, but my instinct says lets figure this out before using the pop up again. Any ideas would be appreciated. If anyone else has had this experience, I would love to hear from you. Thanks for your time.