My TT's 51 gallon FW tank can last me a while. Assuming I brought along a couple coolers of food (to stick in the refrigerator), my generator, a number of bottles of water, 2-3 5 gallon containers of gasoline, I can easily last a week if I'm boondocking in the Texas winter, since I just need to fire up the generator to keep the house battery on. My limiting factor will be grey water and showers. Come May-September and the Texas heat, my limiting factor will be gasoline since I will have to have the A/C on from 10:00 in the morning until 7-8 at night, which means I have to have four gallons of gas per day in a container. Fans won't cut it in 100+ degree weather with a high humidity.
I've also boondocked with the rig winterized, using bags lined in the toilet. Not as fun as a regular flush toilet, but workable.
Dishes, with using plastic utensils and paper bags, I can minimize water used for washing.
Showering is the biggest use of water with my rig, and usually running the grey water tank full is my biggest limiter when boondocking.
The type "B" rig I'm looking at getting, I am considering buying an enclosed trailer just for boondocking. This trailer can handle additional provisions, such as a number of 5 gallon water bladders, a couple "blue boy" tote tanks, storage for bulkier items such as my macerator pump, a mount on the tongue for my 3000 watt generator (so I don't have to fetch LP gas as often), a place for additional cans of diesel fuel, etc. That way, even though the water tanks on the van may be tiny in comparison to a TT, the cargo trailer will provide the added water/power/waste storage to balance things out.