Vintage Air voltage drop

[69 C10 LWB]

It sounds like you have a voltage drop or bad ground to the cab. If you still have 14 V at the battery your alternator is probably fine. If you had a voltage drop on the fan wire your fan would be slow but your voltmeter should not drop. It sounds like you may have a bad connection where the main power wire goes into the cab or possibly at the horn relay if you still pull cab power from there. Could also be a bad ground to the cab.