At about a gallon/hr...
Perhaps a little too generous in favor of solar.
The "common" 4KW Onan approximately burns 0.35-0.40 gal/hr when light-duty powering of common RV items -> not counting a heater, or a microwave, or an air conditioner. The little Honda 1000/2000 class inverter generators of course use way less than this when supplying power for the same items.
(My little 550W Honda is off the charts on gasoline stinginess when battery charging, plus lighting, plus computer/movie use - all simultaneously: About 0.51 gallons every 5.5 to 6.5 hours and it's noise is 54dB while doing this.)