driving tankers, oil from well heads, bulk chemicals, fracking materials, oil well pumps and valve clusters....etc
You are correct about specializing.
I work in the oil fields as a driver / operator. The pay is so much better than OTR or regional and you can work more hours than OTR and get OT pay. If you have to work away from home base for more than a day you get a motel to sleep in plus perdiem.
You have to get used to short bursts of hard work and then hours of idle time,take a nap get paid for it.