Hi Darren
Why do you want to do this? My van has a factory fitted on board tank - note this is not fitted in a locker, but underslung near the axles. On arrival, this requires 3 trips to the water point with the aquaroll and pumping of contents to on board tank each time. Relax because I don't have fill up for 2 or 3 days? Not quite, as the Wastemaster has a lower capacity than the on board tank, so still needs emptying at the same frequency.
Abroad, the water is cooler than aquaroll-in-the-sun all day water. If you are tempted by the idea of having a supply whilst you travel this is not a good idea, since water is heavy and mobile.
In terms of chores done, I don't think there's any gain to be made
In terms of convenience, you fill up less frequently, but spend more time doing it. Unless you fit a water level gauge as well, water will still run ot out the most inconvenient time!