Depending on the btu rating of the sunflower heater you have will determine that. My personal experience is., no I would not use a 20# due to the weight and size. I would use a 10 or even a 5 is enough for the day. I would recommend a heater a bit bigger than what you have. Maybe the next size up buddy heater. With your average temperature around 0 as you stated, bigger heater is the way to go for those temps.
Now.....for the days it's 20 or so degrees, your sunflower should do the job just fine.
Hope this helps.
Actually those sunflower heaters pack a punch.
Mr Heater Sunflower Heater
10,000 BTU/hr.; 12,000 BTU/hr.; 15,000 BTU/hr
Mr. Heater Portable Buddy Heater
Two heat settings: 4000 & 9000 BTUs
Mr. Heater Hunting Buddy Space Heater
6,000 and 12,000 BTU/hr
Mr. Heater BIG Buddy
4,000, 9,000, and 18,000 BTU/HR
I do agree that going with a smaller tank like a 5 or 10lb tank would be best. If your 3 man hub is insulated then it will take less time to heat up and it will also hold heat better. Also if the outside of the fabric is black and the sun is out then that will also heat up the hub a few degrees.