You could use food grade 55 gallon plastic drums as holding tanks. They're inexpensive ($10 to $20) They come equipped with a 2 inch threaded bung hole, where piping can be connected. The drum could be placed horizontally, on an elevated stand, outside of the outhouse, and piped to a commode, thus, providing gravity fed water. Fifty five gallons would provide about thirty-six 1.5 gallon flushes.
The same could be done with a kitchen sink. You could use some type of inline filtration system between the tank and the sink. You could also hook up a shower, and if the holding tank gets some sun exposure, you'll have solar heated water.
An underground filler pipe could be run from the holding tanks, to the pond, where you could connect to an electric, or gasoline powered pump, and suck water from the pond, to refill the holding tank, or tanks. This filler pipe could also be used to drain the system, for winter prep, or to prevent stagnate water.
