I disagree.
Heater fans suck amperage like crazy. The chill at 8500 ft is no joke, it can be miserable this time of year. Miserable enough for a battery to lose enough capacity so as to greatly affect calculations and theory. If it isn't too great a hassle, an extra pair of batteries may mean not waking up to 41F temperatures at a quarter past four.
It's all a matter of RISK/BENEFIT ratio. Females exposed to forty degree temperature going to the bathroom are a most definite risk to health and well-being. What happens when the generator doesn't want to run and the clouds roll in? Unh, unh, the two battery scenario is too risky for me in this case. There are times and places to test theory and this isn't one of them...
JMHO