The answer is going to depend on what you are really asking and what you are using. 

For instance, the quick answer (from http://www.jetcompany.com/grain_steel.html) is about 600 bushels.  That's what Jet says a 22 ft will carry.

The correct legal answer is going to involve figuring out how much weight is on each axle, the bridge weight, the empty weight, etc.   You will need to look at the weight ratings on the truck a well the trailer. Having said all that I think you will find legal is about 550, possible is 650.  First problem is it will probably be difficult to fully load the front axle on this truck.

I forget what the GVW etc was on this truck, other than it was adequate.

But I could never load it full and be legal.  About 10,000 pounds is all I could load on the front axle regardless of how far I slid the fifth wheel or loaded the trailer.  I could get over 600 bushels on the truck, but about 550 was all the axle weights would scale.

So now that we all given our ideas, what were you really asking ?