The domestic bank is 4No. 120 amp hours sealed lead batteries, 2 of them four years old, the other two two years (but same make/model type). I get the 15 min boost into the domestic bank, followed by 2.5 mins into the starter battery. One fault in the system is the cable from the unit to the domestic batteries, for it's length, according to the Sterling manual, is only good for about 70 amps (I think 40mm2 for about 3.5/4.0 meters) so am going to add an extra 60mm2 cabgle to see if this results in a higher charge.
Using my "Smartguage" last week I was 59% (full) at about 12.1/12.2 volts. Running for 6.5 hours got me to 95% full, by which time, and over the last hour, I was only putting in about 12/15 amps, which seemed low to me.