[sourdough]

I had not seen it either. I'm thinking Chuck is right. I don't know exactly how much of those vents would be needed but I figure they built it with some thought of the volume required. I do know that I put a mesh bug screen over the exhaust of my last trailer and had to remove it because that tiny bit of extra restriction caused the limit switch to kick off the furnace so they're pretty finicky. You might try blocking one of them to see but if they're using one grate they can/will use both.

Have you tried sealing all possible points of entry from the outside? Do you leave the doors open? I know I went out to the little barn the other afternoon about dusky dark leaving the gate open. Came back in about 5 minutes later and closed it. When the outside lights came on just at dark saw a skunk wander by. He had come into the yard in that brief time I left the gate open. After scolding it good and proper and opening the gate it left but critters take advantage of any opportunity.