The large onboard tank came about as an option over the 12-13L Jerry cans under the sinks. These ate into the most readily accessible storage areas and are a pain to fill. Somewhere along the line some owners retro fitted an external fill from an Aquaroll as an alternative. To me it should be an either large onboard tank or Aquaroll, not both.
To make more confusing Hymer/Eriba started to offer something called a "City Fill" on the German vans, sometimes after 2012. Why is beyond me. I'm guessing they were trying to keep UK owners of Aquarolls happy as you can use one of these systems next to the exterior water cap to fill the onboard tank, which sounds like your set-up.
I am on my 5th Eriba and only the first had the two Jerry Cans, the other 4 have had a large onboard tank. I have never found any of my vans lacking in locker storage. Unlike some members I have also never had the desire to use an Aquaroll. It is only my opinion, but I cannot see the sense in having to pack away something else at the end of your stay, that could well be dirty and wet, that will need wrapping in something to protect the bathroom cabinets as this is where most carry it. The contents of it on a hot summers day get hot and in the winter could freeze. No it's not for me.

Colin