Our camper has a 20 gallon fresh water tank. I purchased a 30 gallon Aquatank 2. Very lightweight, but reviews and online anecdotes suggest it has good longevity and is tough. Dimensions are 36" x 36" x 14" which makes it a very easy fit in the back of the X. When empty, it rolls up nicely. I think the 60 gallon would work (36" x 48" footprint), but that's about 500 lbs of water in the back of the X. If I'm towing while carrying a full tank of water, that already maximizes rated tongue weight. This setup is 250 lbs and is more manageable.