Insulate Your Roof Hatch!
I was sitting in my living room, huddled under a blanket draped over an oil-filled radiator, and couldn't figure out why my head was cold. I started feeling around and noticed a draft coming in through the roof hatch. The seal wasn't doing its job! Rather than get a new hatch or breaking out a tube of caulking, I took careful(?) measurements and headed off to Wally World. SURPRISE! Wally World had a pre-cut piece of seat cushion foam (22" x 22" x 4") that, although it doesn't completely seal the opening, does a rather admirable job of keeping the cold out and the warmth in. I did need, however, a bit of duct tape to hold it in.
I have an indoor/outdoor thermometer that tells me just how good this idea worked. BEFORE: a 10 degree difference. AFTER: a 30 degree difference. BTW, this will be my 4th Winter Over in Brunhilde.