What effect will heavy use have on the life of our truck?

The way you drive also affects the longevity, maybe just as much or more than towing light loads. I drive like an old guy now and slow down for stop lights way ahead of time. My brake pads went 140k miles. I don't accelerate as fast as most people either the majority of the time. That is how I get 21 pmg out of a one ton diesel.