On how to get water when you can't move the van

I won't ask the obvious question as to why you want an aquaroll ... I assume you want a way of getting water to the van when it's not possible to take the van to a tap. Whether you use an Aquaroll is, as you say, all about where to store it. There is no reason at all why it shouldn't go on the roof - it's light enough. Road dirt will be an issue unless you wrap it well, a top box would be better. My own system is an old 25 liter container, which lives in my top box. I either lug it by hand, or strap it to the back of my motorbike, and 2 trips gives me 50 liters, which I pour directly into the tank (carefully) by pulling the settee off the tank and taking off one of the big round inspection covers in the top of my tank. An alternative of course is to use a funnel and a tube and pour it in to the normal water filler, but then you have to lift the whole 25kilos! I have often thought about getting one of those little shopping bag trolleys for it, but never got round to it.
All Hymer tanks are sited differently, so will be different. I can imagine a 12v pump with a hose to transfer water into the tank from the Aquaroll/container would be quite easy to set up. In the past we have also used the 5 liter supermarket water bottles .... you will be surprised just how quickly you can get a meaningful amount of water into the tank if you run relays and it's good exercise! I would also add that after many years I have found that anything stored on the roof needs a really good reason to go and get it...I always groan when I have to climb up there, especially in the cold or wet. For that reason my 25liter container also can live in the back boot of my 700 - it just snuggles nicely in a corner.