Our favorite time to camp is in winter. Here are some of our tricks to keep our class c comfortable.
Unless you have double paned windows, they also transmit a lot of cold. We have some old wool military surplus blankets we use as thermal barriers on the windows. Ugly but work great and they multitask as picnic blankets in summer.
Herself created an insulated headboard to fit over the window behind the bed. She took half inch rigid foam insulation from Home Depot; covered it in quilt batting and fabric to match the bedspread and slid it into place. No more cold on our heads while we sleep and it looks like its original to our class c.
We also tuck an insulated drape under the cab mattress to cover the access for privacy and to block the cold from the cab. Easy peasy.