There are fresh air ducts that let outside air into your ventilation system. Sounds like you need to find those openings and place screens over them. Something I have not thought of before but will take a look at ours also as we boondock a lot and stay at remote parks. One thing you can do is make sure you have all the vents closed but they could still crawl into the ducts and come in to visit when you use the system later. Good luck and I thought having a Bee inside was bad. Your experience wins hands down!
rocmoc n AZ/Mexico