Cycling 23.000 Km from Alaska to Ushuaïa in 97 Days New World Record

One really needs two sleeping bags on a trip like this. One +10°C and one 0°C. When combined, you can survive -10°C. Those take less room than regular +5°C bag.

BTW nothing is worse than one (too warm) bag on a multi-season trip. You'll get skin problems, because of constant sweating and itching and scratching.