In preparation for a long trip last June, I upgraded from a single grp 24 12v to a grp 27 12v, and replaced most of my incandescent bulbs with LED. I didn't have a volt meter at the time, but, with no TV and as needed light, plus some light furnace use, we dry camped 3 days in Grand Teton. Now I have one of these:
Amazon.com: INNOVA 3721 Battery and Charging System Monitor: Automotive
The rule of thumb is that you don't want to drop below 12.1 volts (measured at rest) or you could damage the battery. 12.6 is considered a full charge.
A single grp 24 has about 75 Amp hours, a grp 27 about 85 and a pair of grp 27 in parallel about 170. A pair of Trojan T-105s wired in series to produce 12 volts has about 225 amp hours and that is what I now have.
After you get a volt meter, try dry camping at home to get a feel for it. Measure your volts an hour after unplugging from house current without any more than the parasitic load. Measure again in the morning without lights, inverters, etc turned on. The furnace cutting on makes the reading plummet, but that is temporary and it will recover. You'll have an accurate reading 30-60 minutes after all loads are off.
Plug back in to house current and see the effect that has. Typically, you will see the reading go to 13.6-14.6, then drop to 13.6 and finally 13.2 (trickle charge).