With the 21, the spacing between the floor cross members is smaller under the rear load area presumably for extra strength so their standard tank will not fit. I have 3 areas under mine where the regular tank fits and have moved 2 tanks around to achieve best weight distribution.
You could consider trying to find a narrower tank, or accept that an extra tank fitted after purchase would need to sit under the chassis cross members rather than directly under the floor.
Another option, especially for use as a grey tank would be to build it yourself using 100mm storm water pipe - can take any shape or length therefore capacity you need.
Remember that whilst the Basestation has a fairly generous pay load, 3 full tanks is nearly 300kg.
What other options are you considering and do you have any 'toys' that you are planning on carrying in the back?
It's a fantastic design and you won't regret buying a Basestation.
I have ticked a fair few option boxes like bunks, jhub, drawbar, 4 grill, harrier air (no ibis), 22 tv, ext ent, ext shwr + light, picnic table, 185lt fridge, 16in wheels, extra wardrobe, shelves, extra 240 & 12v points, bed storm cover. May be a few others which will come to mind as soon as I press send!
Was hoping to add extra water tank to the list but that looks unlikely. I am also looking at an aftermarket rear view camera which I'll install myself.
The main reason for choosing a Basestation is that I see it as a very versatile van.....I have a couple of young boys so will hope to fill the garage with motorbikes (and for myself!)