Sargent Control Panel EC328 - Battery Reading

Does anyone know if there is a delay in the accuracy of the readings on the control panel when the vehicle hasn't been used for some days/a week or so?
My leisure battery sometimes shows as zero volts but all the equipment in the van works and once the engine has been running for a couple of minutes, all battery readings are 12.5v+.
I've also noticed that when I start the engine after a period of c1 week, the petrol reading on the dashboard 
sometimes shows as empty even when full before realigning after a couple of minutes.
Are both these situations effectively the van 'warming up' before aligning itself as everything works as normal?