Think of it like buying a sprayer, are there any small profit centers from you owning a truck?

Would you be able to blend a little more grain if you are having problem with TW or another dock? If you had your own truck?
In your cost figures it really should be eliminate one straight truck, and add a semi. Cost per mile on a straight truck is close to a semi, but double the time and fuel per bushel.
Are there any quick ship bids where you could pick up another nickel or dime?
The grain that goes local, can that be trucked farther for a net gain?

As for repairs, its a total crapshoot.  If you can find a truck that has had decent maintenance, they can be pretty good.  If you aren't able to do some repairs you will be nickled and dimed to death on a semi.  Trailers are very low maintenance, brakes and tires are the main costs.

Figure roughly 5-6 mpg, or 2.1-2.5 kpl with a bigger load.