We made a removable carpet piece for winter use. Installed it over a thick foam carpet pad from Home Depot. Huge difference in temps, plus it feel nice to have carpet in the winter. I also have an approx. 18" x 30" heated rubber foot mat. If you have shore power - awesome!
I am looking into the thin, under carpet heated pads they sell now. Those weren't even available a few years ago... some of them can be cut to size to really get a custom installation.
We could have it better with external rigid foam insulation, but the carpet was easier for me to do by myself b/c DH has had enough "camper mods" and has declined my "offers" to let him "assist" me anymore this year LOL :)