Most Onan generators from that timeframe if it is a model designed for use in an RV will be quiet. My definition of quiet is if you can barely hear the generator running when you are 20 feet away it's quiet. If you are inside of the RV you barely hear the generator, you may feel a very slight vibration but only hear a muffled rumble when it's running. That's what I consider quiet.
Any RVer whether they have a motorhome, van or vehicle towing a TT or 5th wheel can expect 6 to 12 MPG. Most will fall in the 10 MPG range at best.
Also since you are looking for an RV, keep in mind most RVers start off thinking they need a small RV. However the statistics show the typical RVer over the course of their RV life will end up with larger RVs. Ususlly will go through 3 RVS until they get one big enough to truly meet their needs. The normal course is to start off with a truck camper or 15-20 foot travel trailer then move up to a 25 to 30 foot TT, MH or 5th wheel... then land on a 35 to 40 foot MH or 5th Wheel/ TH.