On the original varta batterys they are date stamped on top of the battery post,
For example, 25 that would mean week 25 Year 2009.
09
If that's the case I would say there past there best, as the batterys are wired in parallel the voltage will only be as good as the worst battery,
I have 2 new batterys in my Cali and after 2 weeks standing they still show 100%, But if I was to fire up the heater they would drop to 80% quite quickly and then remain there for sometime,
The firing up of the heater does initially pull 13-15 amps then as things get up to temp they draw considerably less,
Don't forget just because a battery shows 12.9 volts, it doesn't meen its a healthy battery, it's not until you start to load it that it will show its true colours,
I think even with new batterys, You should still give a good mains hook up charge at least once a month, and most certainly just befor you go wild camping,
I went for genuine VW/Varta batterys because where the under seat one fits, it is very tight and I also wanted to keep/refit the rubber insulation cover,Because AGM batterys are very temperature sensitive , also the smart charger is designed to work with 75AH. 420 Amp batterys, x2.
V-tec