I am sure you can rent a motorhome in Colorado during the winter. There are several rental agencies. You can find them on the Internet. Cruise America is one of the well known motorhome rental agencies.
You can check the prices on the Internet. The cost should be less expensive during the winter months. Including the cost for gas, rental of the motorhome and mileage charges, you will probably pay at least $200 to $400 per night. If you are traveling alone, this can be quite expensive, and you may do better by renting a cheap car and staying in inexpensive motels. If you are traveling with three or four others, the price could be a real bargain.
I would be concerned about the weather. You will be driving in the Rocky Mountains of Colorado and Wyoming. There can be a lot of snowfall during February. Some of the roads and mountain passes might be difficult with a car and nearly impossible with a motorhome.
Camping in Yellowstone may not be possible. Most of the roads and most of the visitor facilities in the park close for the winter from mid September until April or May. Nearly all of the campgrounds will be closed. I believe there is one campground in Mammoth Springs near the north entry that stays open all year. There may be a few campgrounds outside of the park that remain open.
You can certainly visit the park during the winter. It is a beautiful place during the winter months. Unfortunately, you cannot drive into the park with your motorhome or your car. You can only enter Yellowstone park by hiking, by cross-country skiing or on an over-snow vehicle such as a snowmobile or one of the snowcat tour busses.