Sort of correct. You're trying to apply a singular rule to a varying set of conditions.
Batteries have two main variances: quality and capacity.
Capacity needs to be measured in terms of Amp-Hours. For example, if I wanted to operate a 50 watt light bulb in my camper for 3 hours, that would require 15 amps. 50W/12V*3hrs = 15. Batteries shouldn't be discharged below 50%. So, assuming that the entirety of my power use on a camping trip is that one light bulb and I camp for 3 days, then I'd need: 3 * 15 *2 = 90 Amp-Hr capacity battery. That allows me to pull 15 amps out each day and still not draw more than 50% of the battery's total capacity.
So, you need to do some math and some research. There are a few things that are always on, like your CO/Propane detector. These have small parasitic draws. Your light fixtures have a known power draw, as does the water pump. If you run the furnace, the fan will draw power. And, so on. Again, you can search on these components and find pretty good consumption data.
Add it up and calculate a daily power need. Then, figure out the capacity of battery (or batteries) you require. You may need more than one.
Let me give you an example. In my 2007 Fleetwood pop up, I had incandescent light bulbs in my light fixtures. Each bulb drew 1.45 amps and each fixture had 2 bulbs. So, I drew 2.9 amps per hour of use per fixture. My furnace drew 4.2 amps. My pump drew 4.75 amps. My water heater drew 0.66 amps. My LP/CO detector drew 0.05 amps.
I used about 1 hour of lights. My furnace ran for about 6 hours per night. I didn't use my water system, so no pump or water heater. And, my detector ran for 24 hours. That totals to about 30 amps in a day. In a weekend, I'd camp for 2 days and use 60 amps. Therefore, I needed a battery with a capacity of 120 amp-hours.
Capacities aren't as readily available as CCA (cold cranking amps) or reserve capacity. You have to search for it. CCA and the other common ratings have to do with starting an engine ... short bursts of a lot of power and voltage. You don't care anything about those ratings. You don't want a "starter" battery for starting motors. You want a deep cycle battery for storing power. That's a nice segue to the second topic ...
Quality. You probably have a really crummy starter battery from the dealership, size Group 24, which is just about the smallest capacity.
What you want is a true deep cycle battery. What you'll find at Walmart, Sears, Autozone, etc. is a Marine battery, which ends up being a sort of hybrid of starter and deep cycle. You'll have to go to a specialty battery, industrial store to find true deep cycle batteries (at least, that's been my experience).
The better the battery, the more resilient it will be to the charging and discharging cycles. Better batteries will give you the rated storage and capacity for a longer period of time. Cheaper batteries may not meet their specs and will wear out faster ... they will all wear out eventually.
Battery state. 12 V batteries do not discharge in an intuitive fashion. That is, a fully charged 12 V battery doesn't read 12 V on a multimeter. Similarly, a dead battery doesn't read 0 V on a multimeter. Full is going to be near 13 V, a dead battery will be 11.5 V or less, and a battery at 50% capacity and in need of a charge will be around 12 V.
With that said, you can certainly use a multimeter to check your battery on occasion. More can be learned here, as the 12 Volt Side of Life
If you use a solar panel to charge, you'll need power of the panel, charging voltage, and estimates on sunlight. For example, if you have a 100 W panel operating at 17.1 VDC, you get 5.85 Amp-Hrs out of it. If you get 5 hours of good sunlight and an 80% efficiency (wiring, non-peak sun, etc.), then you get around 24 amps in a day put back into your battery.
If I apply that to my earlier calculation, it means I'm pulling out 30 amps and putting back 24, for a net draw of 6 amps. If I wanted to camp for 4 days, then I'm only using 24 amps and I'd only need a 48 amp-hour battery ... any crappy group 24 battery would suffice.