Charging woes

I have bought my lovely winchcombe motor home but found the aircon driving  home  was blowing through a straw, I noticed the voltage was fluctuating and so had a new alternator  fitted, but no better, infact seems worse as it's only charges at 12.6v now after a few mins. Any ideas gentlemen?

After overnight, measure starter battery voltage, then post what it is..............

Ok, currently 12.4v
Worth knowing is that I'm currently in france off grid with the radio on so i will expect a drop overnight but I'm reasonably sure I don't have a drain issue anyway, my problem is charging, ie when starting the engine it goes to 14v but after a few mins drops to12.6

Ok so before sun got up it was 12.1, but now up to 12.4, as i say my concerns are about driving charging or the low charging as we drive, I read somewhere about something that controls the charging and tells the alternator how much to charge ?