Check out any unusual current draw before wiring external regulators. It's entirely possible that a malfunction can draw enough current that the alternator can't hold proper voltage, trying to supply enough current into a partial short. Pulling one fuse at a time for accessories until the voltage jumps to normal, is a good way to troubleshoot.
Regulators usually die a hard death, instead of the voltage gradually diminishing. I'd look for a fault elsewhere. I had a bad fan harness once with an intermittent short, which would not blow a fuse, but caused a downward spike to 10.5V. Sounds like something is degrading other than the charging system itself, and loading down the alternator. Or corroding connections.
A bad diode that deteriorates can act this way, but usually it wouldn't last 3 days. A good test is to let the car sit overnight and see how much the battery voltage drops. One bad diode will suck it down overnight so that it won't crank. Regardless, assume the regulator is the least likely thing to fail, and check ALL else first.