Outside wear is generally positive camber or toe-in out of spec.
Cupping is usually balance or bad shocks.
Usually overload will cause negative camber so I doubt the water tank is an issue. Axle could have been bent if someone jacked on the axle tube in the middle section at some point. This would cause positive camber. Road hazard would not likely cause this except maybe going up a curb etc. at an odd angle to bend something.
Some truck/trailer places can bend the axle to realign. Otherwise you are looking at replacement.
Yes could be a bad tire but odd to be both on same axle. Tread separating or belt shifting can do lots of odd stuff.