Rule of thumb for larger RV generators is a gallon per hour to run them. Also, factor in that the generator supply line is usually restricted from drawing fuel from the bottom 1/4 of the vehicle tank (to prevent the RV from running out of engine fuel).
So, if you had a full 100 gallon tank, that means ~75 gallons available for the generator, at 1gal/hour = approx 3 days of continuous run time.
Yes I remember that about the bottom 1/4 of the tank. Thanks for the other info.