I use a synthetic mummy bag for backpacking. It's smaller, warmer, and fits in a small bag. When I take the camper I use a down rectangular bag. It's more roomy and since it's inside I don't worry about it getting wet.
Don't leave it stuffed in the bag. Take it out at camp asap to let the fill expand and form the air pockets that keep you warm. Take it out once you get home also.
Get a good sleeping pad if you're sleeping on the ground. The ground can freeze your ass pretty quick with nothing insulating you (if it's cold) since you'll be compressing all the fill with your body against the ground.
Don't sleep in the clothes you arrived at camp in. There's a tiny bit of moisture in your clothes that can keep you cold at night. Change your clothes or go nekkid.
There's a thousand bags out there to choose from.