For the people who asked, yes, they are single pane windows and they were dripping.
Once I can camp again, my first thing to check will be any potential air leaks. I was sure it had to be something with the furnace, but the more people mention the air leaks, the more it makes me wonder. My kids weren't camping with us that weekend so I'm not sure if I even went into their bedroom. The roof vent may have still been open a bit. Also, I previously mentioned the gap in the one slideout. If cold air is getting drawn in and the hot air is escaping, I bet that uses lots of gas.
Of course, until I get out there and try it, it's all speculation. I will update after I do. Fingers crossed.
Thanks again.