One of my 540s had a battery voltage meter fitted on the end of the roof locker above the kitchen, so it was visible as you entered the van. Providing you weren't seeing stars having lacerated your bald spot on the doorframe.
I never did check it out but presumably somebody had run cable from the nearside rear lounge bed box, where the battery resided, across (presumably under the van) and up into the cupboard under the sink and somehow up into the roof locker. I never even bothered to clock the wires and their routeing and it's only now I'm wondering how it got up from the under cupboard into the roof locker without being visible.
IIRC that was in the '01 van, not the ex-Frantone '05 version.