Since the fluid may not have ever been flushed, there is every possibility that there was a lot of moisture mixed with the brake fluid. In this case it could have boiled WITHOUT actually getting hot enough to smell "hot brakes". Moisture lowers the boiling point and enough water means that it could have boiled under conditions that were not much hotter than they would have gotten with normal braking. You can spend as much money as you want, and change out everything, but I'll bet that just changing the brake fluid will fix your problem. It is a fairly common issue on the P chassis Workhorse.