12v tested but no power

Hi all, first time motorhome owner and forum poster. We've a 2004 Adria Coral on a 2.8 Ducato. There is currently no electrical power. When hooked up or with the engine on everything works and the display reads 14v. As soon as I plug out or switch off the engine there is no power.
I teted 12v at the battery. Ive tested the continuity of the 30 amp fuse coming from the positive thermal and its ok.
Any suggestions? Thank you