I started my 2005 2.3.jtd recently , not having used it for a while (covid). It started ok but I soon noticed no power in habitation. I checked power lead fuse from battery connection which was ok, Checked power at main feed from alternator 14.2 volts, so I assumed my leisure battery was flat. I got my generator out and when I plugged 240v lead into MH ,power supply in MH all came to life,
so i left generator running for 1 hour. All appeared to be ok. Next day exactley same happened but in stead of using generator, I plugged an inverter into cig lighter power came on to habitation psu / charger (Calira). I disconnected inverter and left alone. Anyone any ideas would be most appreciated

Habitation 12v issues can be specific to the PSU fitted in the van; your year will probably have a Schaudt or Sargent with specific model numbers?

However as a general test you could measure the leisure battery voltage without engine running or any 240v power sources connected and then again with.

These should be in the range of 12 - 12.6 and 13.6 - 14.4 respectively depending on the health and state of charge of the battery.