We have an older 31DS, 2011, and have just returned from the cold of Utah, Mesquite, AZ and Las Vegas - where night time temps were down to 32 deg. No problem with the propane heater keeping us warm. Usually turned it up in the morning and the MH warmed up quickly. I haven't counted the vents but however many they kept us warm. I may just count them tomorrow although I think heat output is more dependent on the BTU capability of the heater.
2011 Jayco Greyhawk 31DS with wife & 2 dogs
06 Jeep Grand Cherokee LTD