"The clutch (clutch kit), flywheel, master and slave cylinders, and the lines to the cylinders have all been replaced; some twice."
You’re not going to like this…
Take your mechanic for a ride (you driving), so he can tell you exactly what it is that you are doing which is wearing out your clutches. Yes, you are burning out your clutch. It may be what oldtimer11 says - resting your foot lightly on the clutch pedal when you are not shifting. Or using the clutch to hold the truck on a hill when you have to stop for a light.
But I’ll bet money that it’s something else (or both). I think that when you are shifting, you step on the gas BEFORE you have fully released the clutch, and you are holding the clutch pedal at the friction point much too long. This is a common habit, practiced by those thinking this is how to get a smooth shift. It’s not.
On level ground, you should be able to start from stop and fully release the clutch without stepping on the gas AT ALL. No, I’m not kidding, you should be able to do that, even if it’s not the normal way we drive. It’s all in your feel for the clutch “friction point”.
In normal driving, you apply the gas a split second after your foot comes off the pedal, NOT while the clutch pedal is at the friction point. Same when shifting between gears - lift your foot to the friction point, hold it there for a tiny fraction of a second, and then lift the remainder of the way and get your foot off the pedal. Then step on the gas.
I replaced a clutch on my old Nissan pickup in 1998, not because it needed it, but because something had come loose in the transmission at 303,000 miles and I had to pull the transmission for the repair. But anyone knows that if your transmission is out, you replace the clutch because with the transmission out, it’s a simple job.
After that clutch job, I used that little four cylinder truck on a major renovation and continuing work on rental property. I regularly hauled heavy loads like a lot of sand, or often pulling my relatively heavy utility trailer full of soil or compost, or in one case, pulling the trailer with a bunch of broken up concrete, among other things, all of which put extra demands on a clutch.
Even with that heavy overuse of the little truck, the clutch was fine 13 years later, after 100,000 exclusively around town miles, when the head gasket finally let go.
Hate to break it to you - you need instruction on how to use a clutch.