The easiest way of getting the oil out is to run a hose into the filler and let it overflow with the engine running. You only need a fairly small flow, as the oil comes to the top it will run over the side. It will probably take a long time to get rid of all of it but as long as you have mostly water in it you can use it. If you give it a flushing session every time you come home it will gradually clear it.
Peter
Peter