Mine has a "30 gallon" tank (actually more like 30 usable gallons, tank itself is probably closer to 40, from bone dry to overflowing). Since I plan to rack up some miles on it, I feel a considerably larger tank is in order. I'm debating cannibalizing one from a road tractor in the 100 gallon range.
Since mine's a shorty, I only have ~54" between the steps and wheel arch, so using a longer tank isn't an option. I had considered dual tanks, but the other side has the exhaust routed in front of the tire (I could re-route it, but...) and the air tank (I'm not so into relocating this). So the second tank is out unless I put one behind the drive axle between the rails. It can't go on the right rear because wheelchair lift, and it can't go on the left rear because tool box.
As I recall, a 25" diameter tank, 52" long is close to 100 gallons. (Some quick math shows ~25525 cubic inches = ~110 gallons, but tank makers rate under that for expansion). At ~10 MPG that would give me a mathematical cruising range of 1100 miles, but probably closer to 800-900 comfortably, and realistically probably half that before I'll want to stop and rest, eat, and whatever else.