Using the main engine to charge the coach batteries charges them very fast and ultra-quiet, as the cabling from the alternator to the coach batteries is a fairly good size, the alternator can supply up to around 60 amps at idle engine speeds, and our idling Ford V10 makes very little sound and is free of any vibration in the coach floor, however it takes around 0.7 gallons per hour to idle the big engine.
Using the 4000 watt Onan to charge the coach batteries makes a bit more noise and vibrates the coach floor just a little, but also wastes fuel because this under-loads it so much, as our onboard charger is only the stock 13.8 volt converter. (However our batteries are AGM, so with only 13.8 volts charging them they still charge fast enough for our needs using just the converter.)
So ... we use a little Honda four-stroke ultra-quiet EX650 to drive both the stock converter and a small Sears battery charger (in parallel with each other - for even faster charging of the AGM batteries) to do most of our drycamp coach battery charging. Here's a spec sheet of discontinued Honda portable generators comparing them, which includes our EX650 model, note that at it's full single speed of 3600 RPM it's noise level is only 54dB:
http://www.tappedin.com/hop/html/litegen.htm
