Yes, use the full capacity of the bank in AH so the AH counter will be able to monitor wrt to "full".
However you should not believe the percentage SOC by itself. Your entry of 340AH is for batteries in good shape at 25C/77F and batteries lose about 15% of their rated capacity when outside in the cold just above freezing, eg. That means there is a wide range of "full" but the AH counter does not know what the temperature is.
Also there is the problem of when you recharge the batteries to what you think is "full" you can be at less than truly full, depending on how that goes. That means your AH counter will be out of whack and get more out of whack with each recharge. This will make your "percentage SOC" a joke.
What to do? You look at your "morning voltage" before solar kicks in and jacks up the voltage, and when the furnace is not on so you get the nearest you can be while camping to "resting"--ie no loaded voltage making the voltage lower than it really is. Despite purists saying this is false info--it is "close enough" for the purpose of using the "morning voltage" to get the SOC from the voltage/SOC table for your type of battery.
Now you can compare that idea of SOC with your AH counter's idea so you will see if the AH counter is out of whack.
All that is true for batteries in good shape at their rated capacities. But what if yours are older and not in such good shape? 340 is too high. What is the correct number? You can tell from your "morning voltage" fairly closely.
Say your morning voltage is 12.1 and that means you are at 50% of whatever the real full capacity is. Your AH counter says you are down 150AH from when you last recharged to full and at that time you confirmed it was "true full" by hydrometer SG. (no good if you did not get to true full)
So you can say now that full is twice that 150AH --300 instead of 340.
If it is 35F out you can say -if the batts are still as rated- then the 340 should be about 15% less or 340-51 = 289. If the AH counter actually says you are down 125 at 50% by voltage, then full is 250 and it should be 289 so you know your batteries are not as rated anymore by about 39AH and are at 301 these days at 25C/77F (ignoring taking the 15% off 340 instead of 289 which number you didn't know yet--you can tidy that up if desired)
So now you could enter 301 into the monitor as your "full" and be closer to the truth. It will still get out of whack and you need to do the cross-check with the morning voltage and temperature factor. The percentage SOC readout should be treated as likely bogus at all times.
You are not done yet! The problem of not recharging to "true full" means you need to reset the AH counter to zero (called syncing with some monitors) whenever you do manage to get to True Full.
So monitors are a good thing, but not to be trusted unless often verified, and you pay attention. This can be against the grain for many RVers who are out there camping to relax and not have to think about stuff.
Too bad so sad, but if they just keep looking at their "percentage SOC" reading like it meant something, they can have their camping experience go bad with surprise dead batteries at 3am--which is not so relaxing.
In light of all that, the OP IIRC, said last year or so his four 12s were not in good shape, especially one of them, and he was going to replace them. If that "340" bank is the same bunch of batts, then goodness knows what the real capacity is to enter by now.
1. 1991 Oakland 28DB Class C
on Ford E350-460-7.5 Gas EFI
Photo in Profile
2. 1991 Bighorn 9.5ft Truck Camper on 2003 Chev 2500HD 6.0 Gas
See Profile for Electronic set-ups for 1. and 2.
