Hi Phil,
There is, I believe, one "dual fan speed" furnace for RV's.
I did a heat loss study a few years back (11/14/11) and found that the energy need to keep up with heat loss did not exceed the output of a 20,000 btu furnace. Going "BIG" is only going to be useful for quicker warm up in a cold soaked RV.
I found my results:
I've been busy over the last six months making modifications to my Class C to improve heat loss and prevent fresh water line freeze ups.
My "back yard" experiments results suggested about 430 Btu's per 1 degree C (1.8 f) delta T, or about 126 watts of heating.
Our first cold weather arrived on the weekend (11/14/11) and here are the results, after the upgrades:
968 watts =~3304 btu's 18 C (32 f Delta T) above ambient, 183.6 btu's per degree and 53.8 watts per degree.