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?

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 ?