Hi, There are very few 6000 watt inverter generators. The Yamaha 3000 and the Honda even running flat out are fairly quiet. The 6000 may be louder at 1/4 throttle, which is generally where the eco switch moves the speed to under low load. If the 6k unit is rack mounted (and they are HEAVY) the noise will transmit through the frame of the trailer to the interior.
The lower weight 3000 watt units can be deployed on the ground so that no noise is transmitted via the frame.
Cost of operation may be higher on the 6k units.
I'd add a hybrid inverter and run on the eco setting of the 3k generator.