What is the purpose of the scene in which Frank and Tub stop at the tavern for food and coffee, leaving the wounded Kenny in the back of the truck?...

When Frank and Tub stop to warm themselves up, this demonstrates how little they care about Kenny. Most people would rush a friend to the hospital immediately, but Frank and Tub think of themselves first, leaving Kenny to suffer in the truck alone.

The hunting trip is symbolic of what should be a male-bonding experience based on the environment the three men have been raised in. Kenny bullies the other two, teases them, and plays jokes on them. The three men do anything but bond with one another, until Kenny gets shot. Once Kenny's practical joke goes wrong, and he's shot, then Frank and Tub begin to bond with each other. The ironic part of this is that Kenny is out in a cold truck dying while Tub and Frank are inside sharing their secrets. Frank and Tub are selfish, taking care of their own wants and needs first. I also find it's ironic that the weakest member of the group, Tub, is the one to shoot Kenny, the strongest of the group.

Frank feels the need to open up to Tub and share his fears about deserting his family. At the next stop, Tub shares his secret about his overeating. The two men have managed to reveal their innermost thoughts and fears, probably for the first time, to another man. This bonding experience, however, is at Kenny's experience. The two men are so oblivious to Kenny's situation at the end that they even take his blankets away because he keeps throwing them off.