Nothing worse than having to face a loft ladder when your desperate...
Some of the canal liveaboard bunch carry a hose and pump setup so they can empty in any handy sewer manhole close enough to the towpath, a mate of mine did.
If a PP is used as a seat in a compact shower room beware as there is a void between the two halves that held a bottle of the chemical when new, it fills with soapy shower water and soaks your tee shirt when walking through the boat to empty it and fills you with dread as you hold it as close as possible to contain the leak that you assume is from the tank, finding it's just shower water is a welcome relief.
Or so I'm told, not me honest like...