I experienced the same situation with my 17.5 generator. I was able to obtain an electronic version (.pdf) of the service manual and Skiff posted it on the site under shared articles.
http://www.prevostownersgroup.com/us...S%20MANUAL.pdf
Once the manual is opened (It is a big sucker, 5.37 Meg, so be patient when opening.)go to page 22, look under Voltage Connection, then second paragraph. Apparently, the control point is 240 volts and the two 120 volt levels will fluctuate to maintain this levels.
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Although the 120/240-volt connection may also be used when all loads requires only 110 volts, it should be pointed out that this connection, the 240-volts, is regulated and the lightly loaded phase, or line, will deliver a high line to neutral voltage and the heavily loaded phase will deliver a low line to neutral voltage. The heavily loaded line may have such a low voltage that air conditioning will have more difficulty in starting, and long starting lines may overload generator and trip circuit breakers.
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Hector