If money is non-existant and you ARE operating on a meager budget then you need to rethink. Ultraviolet filtration systems are expensive. Build your own tank. I bought the rocket tank
for my fresh water and need a second one (for 60 gallons total). I have a used black tank but when it warms up, I will be building two custom tanks (vinyl liner on plywood) for my grey and black. Like I keep saying, it really depends on how you intend on using your bus. I live fulltime in mine. I am on hookups most of the time. I have a regular residential toilet in mine. I take showers that last as long as the hot water does. I have a super capacity washing machine. I have a really big sink (8" deep 22X43). I am a water hog. We knew how this bus was to be used and it was built accordingly.
For tank sizing, plug your numbers into this tank calculator
. If you are using a rectangular or square tank, make sure you choose "Rectangular" not "Flat Bottom" under tank type. To make it easy, just type in the height of the tank, the width and then type in how many gallons you want. Click on "Diameter or Length" button under "Diameter in Inches (Length for Rectangular tanks)
" And don't forget that all measurements are in inches except for the gallons. If you are trying to figure out the holding capacity of a piece of pipe then you choose "Flat Bottom" for the tank type and you plug in the number as though the pipe was standing vertical as opposed to the horizontal mount.