I don't understand how 2 GCs won't run the furnace for one night UNLESS, as has been suggested, they are not fully charged when you start or they are toast. I would suspect the former. We used to do several days easily on 2 GSs in our previous rig.
Before you invest in anything, check your batteries with 1), a voltmeter at the battery terminals WHEN NOTHING IS RUNNING OR CHARGING and has been at rest for a few hours. They should read at least 12.6 or 12.7 when charged and at rest. (12.1 or 12.2 is about 50%, which as as low as you should let them go.)
And 2) a good hydrometer. NOT one of those with little floating balls. A hydrometer is really the only practical way to accurately check battery state of charge.
If you are boondocking a lot, you really need to invest some $$ on a reasonable solar system. For 2 GC batteries, minimum 120 watts, up to about 360 or 400 watts. This will ensure that on a reasonably sunny day your batteries will be fully charged, or nearly so.
But first, check those batteries.
and second, check your charging methods.
