We also have an onboard Onan and have purchased a portable Champion 3.1K inverter genny to power our RV since the Onan is way overkill most times, as well as noisier and less efficient. Although we don't run the Champion genny all night, we do run it while watching a late night movie and run electric heaters to get our rig very toasty before heading to bed.
We then set the propane heater for 55 or so and it may kick on a time or two during the night, early in the morning before we rise I will use the remote start on our Champion from the comfort of our bed to get our rig nice and warm for when we finally decide to roll out of bed.
Keep in mind this practice is only done when we are boondocking in frigid temps with no RVing neighbors anywhere around, we do several of these trips each year. There is a 30 amp outlet on our Champion and we just plug our shore cord directly, amazingly it still has plenty of residual power for other items in our rig even with our AC unit running.