We have used a 3.5 gallon Rotopax for our 2019 Transit since it was delivered in March, 2020. We store it behind the Aluminess Deluxe Box on our rear bumper with a locking mount for security. I got the idea from Fitz’s Sprinter build on this Forum, although he uses a water pax and fills it with Jack Daniels
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The SAFETY Eco-spout is a PITA. I use a flexible self-venting spout that you can purchase separately from Rotopax.
I have used 6 gallon jerry cans for water, but to lug around 50# cans to refill our water tank is a chore for me. I haven’t used the siphon hose mentioned above, but the one referenced is a gravity fed system. There are battery powered pumps and hand pumps that I am aware of, as well. Because of the weight issues, I switched to 3.0 gallon containers to manually fill my water tank when needed.