This still doesn't add up for me. The voltage is being metered at a 12v plug in inside the boat. The 48w self regulating panel is also run through another regulator and hard wired to one battery which is combined with another via a combiner switch. (standard A both or B switch) If I measure the batts in the dark separately I get 12.6v. If I measure them separately during daylight, I get a charge (normally well in excess of 13.3) on the battery that is hard wired to the panel and 12.6 one the other. When I combine them I get the higher voltage reading. Although my consumption is low, I honestly think it is more than 15 amps per day (your estimate) and my panel seems to always keep my batts topped up just fine. One of my lazy lying around the boat things to do is watch the juice start to flow from the panel right as the sun pokes its head over the horizon.
There is a tide in the affairs of men,
Which, taken at the flood, leads on to fortune;
Omitted, all the voyage of their life
Is bound in shallows and in miseries.
Shakespeare, Julius Caesar IV, iii, 217