Gary,
If your generator is noticeably changing engine speed while running, you have a generator problem. The Onan 10K and 12.5K generators are a constant speed generator that is designed to operate at a constant 1800 RPM. This RPM setting is necessary for them to produce a 60 cycle AC sign wave current. This is unlike the Onan 7.5K and 8K generators that are constantly varying in RPMs with their load, because they are actually a DC generator that produces 60 cycle AC with a built in inverter.
Gerald