From Onan 5500 Spec sheet:
Gasoline Generator
No load: .3 gal/hr
1/2 load: .6 gal/hr
Full load: .9 gal/hr
So if you want to run continuously with a no load for 48 hours, that's 48 * .3 = 14.4 gallons. Of course there's always some load: a couple of amps to run the power converter to charge the battery, for example. The more electricity you use, the greater the load and the more gas used.