How long will 26 gallons last? (Also, a power question.)

I have a cassette toilet in my RV. The capacity is only 6 gallons so I have a very good idea of how fast it fills. Two of us can fill the tank in 2 days or less. So roughly 1.5 gallons/day/person. The volume depends on the efficiency of flushing. My system is very good.

To last that number of days with 2-3 people you will need to use public toilets except for at night and you will need to be careful about flushing the toilet.