I've been pulling wrenches on commercial trucks for 10 years now, mostly dealership shops, but the last year and a half in a smaller fleet shop with roughly 40 units. Some days I still enjoy what I do, but more and more I wonder what I should do when I grow up.
Pay is alright, but working lots of saturdays is the norm since weekends are typically when the trucks are off duty, as well as some shift work in most places. Its hard on the body as mentioned, and a big investment in tools is necessary if your going to do it seriously.
Off road equipment techs do make a bit more, and typically easier to get monday -friday or a 4 on/4off type shift.
Certainly no shortage of work in either field around here, if you are good at what you do. Which brings me to my last point, don't enter the trade for money and glory. Enter it because you have some mechanical apptitude, want to learn more, and want to do that type of work. I've seen far too many green apprentices fail miserably because they were there for the wrong reasons, with no natural ability. Your uncles' or cousin's shops would be a great place to try it out before proceeding with school IMHO.