An interesting thread I have never seen this arrangement sounds like as you would find on a touring caravan, but with an additional interior tank ?
I assume the exterior offers the whale pump 12v electrical connection ? If so use an aquaroll or food grade gerry can with a spare whale pump to transfer to the interior.
I have a CAK tank on mine which flls via a hose exterior or have the option of lifitng a seat and filling manually.
if you get time pics of the arrangement would be perhaps useful to others and those curiouslike myself
Channa