We have the same brand of service bed on a 96 dodge at work. it pulls a single wheel tandem dump trailer all the time with up to 5-6 yards of rock in it just fine.
But looking at it again, the bumper on it seems more built into the bed on ours, it doesnt hang down low like yours does. If you can, load it up real heavy and see how much it flexes the bumper, that will tell alot better than a picture i think. But since it has a 2" ball insert i would suspect its good up to at least a V5 rating. 500/5000
Also, keep in mind the rust, looks like its getting at it pretty bad.