Actually, I may have mis-spoke concerning the 12.7-8v "surface charge". It's more likely a case of the chronic undercharging resulting in progressive capacity loss. So, even at 12.8v, your battery would be like a much smaller Ah capacity battery, and would appear to discharge much faster than expected.
Those who do 50-80's regularly see this all the time. At some point, the bank must be brought back up to full capacity. At 13.6v it could take weeks of charging, not hours, and even then SG levels would be low. And in all likelihood, there would still be sulfation and capacity loss. Check out the battery manufacturer's recommendations and guidelines for your particular batteries, and try to match up an appropriate charger to them.
