generally these large fleet customers enter negotiation for contracts with the manufacturer (the dealer is merely the middle man) They sign on to purchase a certain number of trucks over a predetermined period of time. The training for thier own techs is an option that would be negotiated at the table and the dealer would be required to supply them with said training and any diagnostic equipment they might need or the manufacturer may elect to train thier tech's at the factory. Probably whichever is the cheaper side of that.
And in alot of cases they add-in a maintenance agreement for an extended warranty that would be either a power train warranty for lets say 5yrs , 10k hrs or something to that effect.

Of course this all depends on what the manufacturer is willing to do to get the contract. If the customer is large enough and buys enough trucks they may be willing to do most of these things but it is not done by all manufacturers for any customer.
They would have to sustain a substantial profit from the transaction before they would even consider it i would think.

As for the small company with lets say 20 to 30 trucks? , i don't think they would even consider in-house tech training, they would force the customer to lean on the dealer for support.

i know of one incident where a rather large company here had thier own techs, they did get factory training and even got the diagnostic materials they needed but were eventually restricted on the service information because of a security leak that was allowing many people to access thier online website for the manuals and service info that were unauthorized. (yes they were handing out the access info to other unauthorized people)
The factory cut them (and everyone else that wasn't a dealer out of the website and sent them just basic software that ran directly on thier laptops or desktops and severely limited thier ability to get the information they needed to do thier jobs.

So in a nutshell any manufacturer that allows large company's access to this information are just being gracious about it, there is no guarantee it would happen.