It is not safe for me to store anything on the roof, especially if it has much weight. I find it too easy to lose my grip and/or footing climbing with one hand. I do believe the younger you are, the easier it is.
On the fuel tanks, for many years the tanks were behind the seat. They were moved outside the cabs in the late '80's as I recall. GM moved theirs outside the frame, and vulnerable to side collision. A doom and gloom company staged phoney safety test showing outside the frame explosions. They were to be proven fraud test. Ford, as I recall put their truck tanks between the frame rails. In the rear I think. Greater protection except possibly rear end collisions. Eventually most if not all fuel tanks are made of plastic, with no metal shield. The Pinto automobile had the fuel tank in the rear, and Ford was sued due to rear end collisions and fuel tank explosions.
If it is more safety you are after, you can be safer by driving a diesel fueled truck. Diesel fuel is not as easy to burn as gasoline. If the conversion was not so expensive, I would convert my Honda to propane, and probably store the tank on a ladder rack.
I always have at least two tie down methods to secure my plastic gasoline boat fuel tank. The boat tank is much thicker than plastic fuel cans. I have steel NATO type fuel cans now.