Note that a 4000 watt 60Hz Onan, from the chart above, consumes about 0.3 Gal/hr with no load, and about 0.5 Gal/hr at one-half load.
Our Itasca RV is only a 24 footer, so it's air conditioner is a 13.5K BTU unit. I estimate that this air conditioner loads the Onan at about 40% when running steady - but air conditioners usually are not runnning constantly - at least the power-consuming compressor portion of RV air conditioners cycle ON and OFF. So ours is probably using 0.36 to 0.38 Gal/hr when powering our 13.5K air conditioner in all but the very hottest and humid conditions, during which it might be running non-stop and maybe consuming 0.40 to 0.45 Gal/hr. I use Mobil 1 EP and a special spark plug in our Onan in an attempt to help it maybe consume just a bit less Gal/hr and run just a bit cooler.
FWIW as a side note, once we used BOTH the Onan-powered coach air conditioner and V10-powered Ford cab air conditioner in the Texas Panhandle during August for a fast coach cool-down after being outside rockhounding in order to avert heat stroke and have a comfortable lunch!
