Furnace power draw

Second battery and 150 watts of solar.
The furnace electric is fairly efficient as is.

Supplement with a propane heater with adequate ventilation. No, the adequate ventilation does not negate the extra heat from the Mr. Heater type propane heaters. If done properly and with adequate ventilation, this can dramatically decrease furnace run time.

Or, just really awesome down or synthetic bags. Or both.

Fan speed is critical to keeping furnace sail switch closed.......if it opens furnace will not fire.

Atwood does have couple different 2-stage furnaces 'Excalibur Series'
They are ducted models...not direct discharge

BTW, you can use one of these to dim your LED lights as well.

You could install a PWM in line with the blower fan. A PWM takes the positive voltage and pulses it on and off thousands of times per second. By adjusting the length of time it is on compared to off, the fan will run slower or faster with solid on the highest speed and solid off the fan is off. PWM's are used in 12 volt trolling motors so the operator can control the speed of the boat. You can buy a PWM off eBay. Here is a link to one;
http://www.ebay.com/itm/12V-40V-10A-Pulse-Width-Modulator-PWM-DC-Motor-Speed-Control-Switch-Governor-US-/261621894215?pt=LH_DefaultDomain_0&hash=item3ce9e14c47

They are pretty cheap.

For a little more money, you can get one that is in a box;

http://www.ebay.com/itm/9V-60V-DC-Motor-Speed-Control-PWM-HHO-RC-Controller-12V-24V-36v-48v-Max-600W-MUK-/171494177223?pt=LH_DefaultDomain_2&hash=item27edd979c7