What Bruce said. Twins or a single (or triples/quads for that matter), the WOT RPMs should be right in the middle of the recommended range. 5200-5600? Shoot for 5400. Prefer to go higher than lower...
What are you at right now?
As far as the charge output, I would check the voltage at the batt at 2200, see if it coincides with the voltmeter. If so, you are probably looking at the voltage regulator. But only around 2200? What does the voltmeter show at 1500? At 4500? Inquiring minds want to know...