No power when disconnected from EHU

Hi Everyone

Can anybody tell me why my 2009 coachman amara 640/6 has no power at all when it's disconnected from the EHU? There isn't even a flicker of life in the control panel. The original battery failed last year so that's been replaced and the new one is fully charged. (With the failed battery there was no power even on EHU until at least 10 minutes after connecting.) Everything works fine when it is plugged into the mains now. Confused.