Far more important than the location and number of vents (which vary widely from coach to coach), is the size in BTU's of the furnace itself. A 31' Class C has a lot of interior volume and unless the furnace is something north of 30,000 BTU's it will have to work awfully hard to keep the coach warm. {A 31' Class C will also need more than a single 13,500 BTU AC unit to keep it livable in the warmer months.... but I digress}.
This means that the run time will be excessive which will pull down your house bank substantially as the DC furnace blower is a very large draw. Our little Nexus Phantom 23P (24' Class C), came with a 35,000 BTU furnace and 4 vents and could easily function as a sauna (well, almost...). :W
Rarely if ever does it stay on more than 5 or ten minutes before shutting down after reaching 68 to 70 degrees. We have camped in temps down into the 20's with no issues and I have yet to even turn on the tank heaters. Another tip is to close off the cab area from the coach with a heavy curtain or blanket. Also adding insulated windshield shades or external covers for the windshield and door windows will go a long way towards keep the cold out.
:C