LP generators fuel consumption

Onan has a bunch of literature online. Here is a PDF of the spec sheet for the QG 5500 LP generator, which would appear to be the model you're interested in. It lists propane consumption as follows:

No load: 0.4 gal/hr (1.8 lb/hr)
Half load: 0.8 gal/hr (3.3 lb/hr)
Full load: 1.1 gal/hr (4.6 lb/hr)

I cant give you consumption numbers but I had a Champion 3500 that I converted to propane and it was a pig. That may not be a good comparison to an Onan but I have heard others complain about consumption with propane generators. There are gas stations on every corner but not propane. Propane can be pretty cheap in some areas and double the price in other areas.

Some reported his generator using 0.5 gal of propane per hr.
Generators are not equal. My 2004 Fleetwood come with inverter generator.
Those save lot of fuel, especially at smaller loads, have lower noise and vibration.
If yours is 5.5 kW generator running at constant 3600 rpm, that can be real gas guzzler.
I think only reliable information source would be manufacturer - if available.