If you can't take the SG as with AGMs, then the way to get the truth is by a load test using the 10 hour method. (I am unsure if garage load testers tell you what you want to know on an RV house battery)
You run a constant load (as best you can) so the Trimetric shows a draw equal to your 20 hr rate. eg 232 AH bank it would be 232/20 = 11.6amps.
If all is well, after 10 hr (from as full as you can get them) at 11.6a, you should be at 50% SOC. With an hydrometer you can stop at 1.190 or whatever you battery spec says is 50%, but with AGMs you have to do some stop and start to try to get your bounce back at 12.1v or whatever your battery spec says is 50%
I find when running such a test, that when battery loaded voltage on the 20 hr rate gets to about 11.6 it will bounce back to 12.2 after an hour. Maybe go to 11.5 and then let go it should get to 12.1ish.
Whatever, does not have to be exact. Ok so how long did it take? If 8 hrs instead of 10 (quite possible with three-year old batteries) that means you are down to 8/10 of 50% (116AH) so you have 186AH capacity instead of 232. (Assumes test run at 25C temp)
