With a 55 gallon gas tank on our Axis we really don't need to carry extra gas--we can go much further on a tank than we can without taking a break...
As far as water I simply throw a couple of these 6 gallon Igloo containers in the toad...
https://www.walmart.com/ip/Igloo-6-G...0?athbdg=L1200
They worked great for our 3600 mile trip last summer.