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Converting to an alternator is avoiding the problem, when the vehicle was new this was not a problem, the generator system provided many years and miles of service. Is the dome light controlled by door jamb switches? The jamb switches, switch the ground portion of the dome light. Is it the original wiring harness? What is the voltage at the battery at idle? What is the voltage at the battery at 1200 or so RPM with no load on the electrical system? What is the voltage at the battery at 1200 RPM with dome light on? What is the voltage at the battery with the headlights on? Is the belt tight on the generator? How old is the battery? How good are the connections? Is the chassis ground good? The ignition system appears to be deprived of voltage which is causing the RPM drop in my opinion. Could be as simple as a set of brushes on the generator, or a voltage regulator, or a parasitic drain through the ground on the jamb switch which could involve some determination and some working electrical knowledge and GOOD volt ohm meter to track down.
There a lot of variable that could be causing this, and an alternator is not going to fix it just hide the problem.
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