You can't get a true "resting voltage" while camping, but you can get a "good enough" version of it by using what I call the "morning voltage".
Take your voltage in the morning before solar can start to raise that voltage is the first thing. Next thing is to wait till after the furnace has quit for a while and before it cycles back on, and turn off any lights and fans too. Never mind the little parasite draws.
Ok so that is your "morning voltage" and you can use the usual voltage vs SOC tables for your type of battery. some 12s use 12.2 for 50% Trojan uses 12.1, and so forth. AGMs have higher voltages per SOC. Hard to pin them down, but mine are about 12.4 at 50%. Whatever.
Now the other voltage you see is the "loaded voltage" and this depends on the size of the amps draw at the time. The higher the amps the lower the voltage. So there is no fixed amount like you were asking about. You can get an idea of the voltage drop to expect for what you have "on" with your rig after a time using it.
Then if your voltage does not "match" what you would expect, you can go looking for what is wrong. This is good practice, because you need to be able to "tell" when things are "not right".
If you had a Trimetric, you could cross-check between the AH down from full, and the "morning voltage" to see if they "match" as a good way to see if all is well.
