If I was going to need it I carried a 25 gallon black water tote in the truck bed when I had an 18 foot TT, the black tank was only 20 gallons.
My current TT is 28 feet and I have never needed it. The black holding tank is 40 gallons. I have used the 25 gallon for grey water with a portable sink at events I have attended and did some cooking for my group.
For extra fresh water I have several square Reliance Aqua Tainers in 4 and 7 gallon, 3 - 15 gal white poly drums and a 50 gallon white poly tank. Carried the 50 gallon tank or 15 gallon barrels in the truck bed and Aqua Tainers in the truck bed or inside the TT's.
If I tent camped I would take which ever containers I would need in the truck bed or 5x8 utility trailer I took along, including gas cans, including an 11 gallon black tote and porta potty.
All depended on what I needed or thought I might need.
