Voltage @ cruise

You should not be "going through" voltage regulators. It may be that the field in the alternator is partially shorted and drawing too much current in the field circuit. Of course in today's world, "Chinese" parts can be ungood

Make sure the regulator is grounded to the battery. Check this by:

With the engine running at fast idle, stick a meter probe RIGHT ON the regulator case, the other probe RIGHT ON the battery neg. post. You should read very little, the less the better. Anything more than .2V (two tenths of a volt) means you don't have a good ground in that path.

Do the same, hot side of circuit. Stick a probe right on the bat. positive post, the other probe right on (close to the reg. as you can get) to the ignition supply. Once again, the less voltage the better, zero is perfect, and more than .2v is too much.

If you have a high reading here, it indicates poor connection from the regulator--ignition feed--through the bulkhead--to the ignition switch connector--or the switch itself, or maybe out through the other side of the switch and back through the bulkhead to the battery.

It is also possible that you have boiled the battery enough that you just plain ruined it.

Cheapest way to check, if you have a couple of vehicles, is to temporarily swap batteries with a known good one and see "if the problem moves."

15 Volts is "getting high." It may be that you are getting LOW IGNITION voltage to the regulator and ignition system, as the tests above should show. In other words a bad connection from the switch may be giving the ignition and the regulator both low voltage compared to the battery, and this will cause the VR to run high.

Some VR's are wrong, too of course.