We had a 1983 E350 that used a simple 60A alternator with external regulator mounted near the battery on the passenger side. That alternator used a nut to secure the main charging wire coming out of it.
Ford had some alternators around then, with an internal regulator also, but the way you describe the sequence of the work, seems you have separate alternator and regulator.
But! Ford had some alternators that used separate regulators AND had push-on connectors instead of the stud-with-nut I mentioned above, and those were troublesome. The connector was troublesome, and so was the charging lead within the wiring of that connector. That's where I'd direct my suspicion.
I looked on RockAuto.Com and it seems your chassis might have an alternator with push-on connector. Can you verify that? Could you post a couple pix of the Alternator, and the Regulator?
Also, verify from the Ford label in the Driver Door Jamb, that the CHASSIS is a 1988. Many coaches in a Coach Model Year are built on the previous year's Chassis. That can affect parts look-ups.