2003 Symbol Hab/Vehicle battery switch query

A brief query, parked up, watching telly with the battery supply switch to 'Hab' it seems to drain both Hab & vehicle batteries according to the battery condition meter set in the switch block above the back door.
Now, telly went off after an hour with low voltage, but the engine starts ok so I think the Hab battery is jiggered (age unknown). 
But surely when set to 'Hab' the engine battery condition should be untouched ?

(Background - SWMBO went off for a winter fell walk, I parked up by the beach + stormy sea & watched 'Das Boot' Ace eh !)

Comments appreciated
T.

It has been 18 months since I sold my 2002 Symbol but I can remember how it works.
As supplied as standard by AS, the habittation electrics are powered by (and drain) the hab battery only, regardless of the switch position.
The switch governs which battery the mains charger will charge, and not which battery is drawn from.

HTH D.

Thanks, that makes sense, I'll inspect the hab battery & replace as necessary.
(Just making sure the vehicle battery doesn't drain when watching telly etc. when in the wilds)