I witnessed an extremely rectangular "tank" with 1/4" walls try really, really hard to become round at what ever a Thomas 12 VDC compressor can put out at max. They abandoned the idea after that.
To me, 125psi of internal pressure on say... a 2" square 1/8"wall tube is a very small effect.
The amount of time it would take to rust out a bumper from internal condensation would, in most cases, be longer than the useful life of the truck. It's no different than the tank on an air compressor. And in Phoenix, where the OP lives, the problem would be pretty insignificant. I live in an area with a very damp climate, and my 40 year old air compressor tank has yet to rust through.
The end caps will be wanting to take on a hemispherical shape, so the welds around their perimeter will have all sorts of bending and shear stress' on them. They'll be getting pulled 6 ways from Sunday just by the air pressure.Aren't the forces on the end caps relatively much the same as those on a similarly sized tube?
I just have trouble with the concept that a 2x2x.125" square tube has that much trouble with the same pressure being handled by my cheap air pig that is like maybe 12" diameter and made of... 22ga?