It's the correct fuses, and it never blew on me.

As for the test, I did run it a idle, with full load (lights, blower motor on high, radio), and it droped as low as 11.75V, then I reved up throttle and the Voltage came up to 12.5 - 13.0V range. I could see the headlights getting brighter as I did this.

The only thing I could think is that the belt is not tight enough, or it slips on the pully???

I ran the same idle test on my 1987 300D Turbo, and had no problems, constant 13.8V. And the best tension feels the same.