Properly installed at the factory an Onan is not emitting 70db of sound. No matter which brand of paralleled 2K watt inverter generators you use they will sound louder when called on to provide max wattage, as noted you will need three 2K watt generators to reliably run both AC units under most circumstances such as high elevations and extreme temperatures.
My vote is go for the onboard Onan, we love and use ours heavily, over 1K hours and a decade of reliable, one button operation thus far. We did purchase a Champion 3100 with remote start being babied by the convenience offered by our Onan, it easily runs our rig's AC unit on eco-mode and at high elevation. Pairing a couple of the Champion 3400's would be my second choice.