Thanks, everyone. I have been trying to figure this out for months, and posted to see if anyone else had run into the same sit, but I guess not.
For clarification, my frozen meals are home-cooked, the van is small so 'half the carpet' means a few square feet, and I have no AC. I can feel warm steamy air venting out the bottom front of the microwave when it's in use, but have not yet tried moving it to a tray to see if there is also dripping going on. The windows in the van do not fog up.
I'm glad to hear this is an uncommon problem.