I Googled "Onan 4000 Micro Quiet noise and vibration". Turns out the Onan 4000 is not an inverter type generator and runs at 3600 rpm all the time. Some say the noisiest part is the cooling fan, more so than the exhaust. Some say the vibration and noise inside the RV is dependent on the vibration isolation mounting and the noise insulation in the generator compartment provided by the RV manufacturer. The Onan 5500 runs a little more slowly at 2400 rpm. The portable inverter generators will run at a speed proportional to the electrical load.
I'd say built-in for convenience and portable inverter generator for the most comfy and quiet campsite experience. Can't have it all, I suppose.
I couldn't find a toy hauler with a built-in generator that would meet my floor plan (front bed, three dirt bike capacity) and weight (1600 lb. tow vehicle payload) requirements. I had to go with the portables.
Some of these might help.
