Re: hacking a grey water tank
As suggested, you need to read basic plumbing construction. Normally size for inlets is around 1/2 inch, and Drains 1 1/4 to 1 1/2 or a mixture of both, as many bathroom drains are the 1 1/4 and the kitchen 1 1/2 . Either size will work well in an RV. You also need to consider buying a 12v pump to between your fresh water tank and the sinks.
The grey water tank discharge should probably be at least 2 inch or so. If you are going to have both a black and grey tank, you really should either go buy a used "T" set up that connects the grey and black tanks together with shut off valves for each side, and plumb up both of the discharge in these systems with 3 inch plastic pipe, as that is what the RV's use. These parts can also be bought new, or sawed off of a rv in a wrecking yard..(hopefully the tanks are empty)
So you need: fresh water tank, pump, maybe hot water heater, something like pex pipe, sinks, shower, toilet and connections, grey and black water tanks, as well as all those fittings. You will also need to vent all of the tanks, and sinks...
Jesus Christ... Conversion in progress.