i dont have a transporter, but i do have a mercedes vito dueliner i used to have a 400 ltr upright behind my seats for quite some time, but then decided i wanted more space so i took the seats out and made my system easily removable. ii know use a 650 ltr tank.
i did also toy with the idea of placing this behind my rear seats, however i didnt like the idea of 650 ltrs over my back axle. hope this helps.