If it would not be that you have checked I would say that the belt/s were slipping on the pulley.
This is a situation where an inductive Amp Meter would come in handy. It takes a reading from the magnetic field around the wire. You slide it over the wire and do not have to wire the meter in series with the circuit you want to measure. The problem is I have not seen them in any of the local auto part stores.
The idea would be to find out if your alternator is maxing its amperage out put. If it is maxing the amperage out put and the voltage is low it may be the Alternator is doing the job but something is using up the voltage.

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84 300D, 82 Volvo 244Gl Diesel