Onan is a very good name. They have 1800rpm units that are quiet and last along time, 3600 rpm units generally are louder (many times by alot). I would go with gasoline, can buy it everywhere, propane not so easy.
My preference is Onan Emerald series. Emerald comes in ( BGD, BGE~4-5.5KW, NHE~6.5kw) or early Marquis with horizontal shaft (BGM ~5-5.5kw, NHM ~7kw). They can be had fairly reasonable and don't have the problems the newer ones have.
They also have electronic voltage regulators ( it prevents high voltage overshoot when a big load shuts off). They do have an issue if you don't run them ~once a month, slip rings get dirty and can kill voltage regulator. So have to clean slip rings before running if stored not runnning for months. But it's what I have, I chose it because of the voltage regulator. I'm not familar with your RV.