Looking for used 3500 diesel dully, to pull a 5th wheel currently looking from 2014-17 wondering what are the best years that I should be looking at?

I maintain my vehicles (+400,000 on my Toyota Corolla) what are the main things to look for/check to determine how well a truck I am interested in has been maintained?

I can I determine if the injectors have been replaced.

Any information is appreciated.

Morgan-