non charging ford cargo 0813 horsebox?

Got a horsebox that dosent charge,changed alternator,now I only get 14.5v at back of alternator when my isolator is in the isolate position,I get this readingbetween battery + and chassis earth isolator side aswell,but not to batterys because of being isolated.The rest of the time I only get 12.91v to the batterys. Have cured previous battery drain from tacho and lack of charge because battery light was disconnected.any help appreciated. Thanks in advance