But I think the 400 rpm drop is excessive and that should be sorted out first.
Not knowing what fans are on there exactly it’s hard to say, like on my car my contour fans pull less in the low speed that comes on first than what they’re capable of at their high speed, but I would expect the idle rpm drop to be more like half what the OP has. There will be a drop, the alternator is loading the engine more with that amperage draw from the fans, but 400 rpm seems excessive.