Just as a reminder of something so obvious that probably many already "know it", but ... the large built-in constant speed RV generators consume fuel in varying amounts versus the loads on them ... just like an inverter generator consumes more/less fuel when it goes up and down in speed in response to the loads on it.
What this means is our constant speed 4000 watt RV generator when delivering, say 1000 watts, most likely consumes only a relatively small amount more fuel than a 3500 watt inverter generator does when delivering 1000 watts.
Regarding generator noise, the inverter generators are only "real quiet" when not providing much power. Another issue not talked much about is: At least with respect to our RV's built-in Onan generator - it also makes less noise when it's delivering loads less than it's maximum.