Originally Posted by bikendan
furnaces are the biggest power drains of anything on a RV.
plus there are a number of other power drains, that you don't see.
i don't think 100 amp hours is that good.
i think you should go to two 6v golf cart batteries.
The whole 6V vs 12V debate is topic unto itself - but just going to 6V doesn't automatically increase your amp hour reserve. There are some other benefits from the 6's, but we have had good luck with two 100AH 12V batteries wired in parallel.
Its the capacity of the batteries, regardless of voltage that is going to make the difference. Having 2 batteries at 100AH each is about what you would get from a similar pair of 200AH 6 volt batteries in series. Not sure what model furnace you have, but typical draw is probably 8-9 amps. And that is a big drain.
If your Nautilus batteries are by Exide, they make several types under that name, including deep cycle, cranking, and combination. Seems confusing that they do that, and now most deep cycles seem to have CCA ratings, which is also confusing when you are trying to find a true deep cyle. The type of battery and the age/condition will impact how much usable energy you get.
If you have a meter, it would be worth checking the draw when the furnace is on. Knowing that would get you to a maximum theoretical run time to compare to what you actually get.