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

There usually is a water gravity fill somewhere on the outside of the RV but, not always. You can add jugs of water, we purchase 2.5 gallon jugs BUT that water has to have someplace to go when used. Some use used dishwater to "flush" toilet to extend the water.

40 gallons sounds like a lot but unless you are VERY careful you can blow through that. NO running water for teeth or dishes, stop every time you can and use low flow water for rinsing etc.

GOOD LUCK! This is an adventure to be sure. Enjoy the adventure.