The fat truck driver myth?

Personally, it depends on the assignment. Worst for me is tramping. Best for me is local builders yards flatbedding or similar.

I’ve heard plenty drivers say you get mentally knackered not physically. That makes it harder to do any exercise especially after a 12 hour stint in the saddle.
For me, I fasted and went down to 13 stone. I’m tramping now and up at 15.
In terms of weight (BMI) and fitness, you’ll have to play tunes on your drives. Some people never seem to put on weight, some of us do.