Welcome to the forum Brian.
The need to recharge the batteries at 12.3v is only valid when the batteries have been sitting with no charge, or discharge for a long time. (It is a resting battery voltage). Even then it is a crude measurement that needs to adjusted for temperature and battery type.
12.3v does sound a little lower than I would have expected for the parameters you have described (15A discharge on a 1200AHr battery bank @ 88% SOC), but it is not far out. Often the voltmeter is slightly miscalibrated or it is measuring slightly lower because it is reading some voltage drop in the wiring
If you are concerned a capacity test is next step. Some electrical
experts recommend a test like this once a year, but it is hard on the batteries.
At 1 year old the batteries would have to very abused to be in poor shape.
Some voltage measurements at the battery while charging
would also confirm they are being correctly charged. In theory SG measurements are better, but it is easy to make erroneous reading when doing this and confirming the correct charging
voltage profile is a good double check that the batteries are being charged correctly.
Also remember when charging via the generator you are probably stopping at around 80% SOC (the final 20% is very slow). So you only have 360AHrs before the battries are down to 50%. If your average draw sailing is 15A you will need to run the generator every 24Hrs.
Finally, 15A is large average draw for your sized boat. Some simple power conservation steps would be worthwhile, if this is your average draw sailing.