Build an out house on wheels with a regular toilet and a pipe coming out the back turned down into a homemade septic tank which will be a 55 gallon plastic Barrell filled with # 3 rock and holes drilled into the barrel. Use the outhouse for storage in your garage until the event happens and then roll in out to the edge of you driveway and install your barrel.
An out house for visitors gives you the option and time to warm up to the idea of strangers in your compound.
To use the outhouse you have to go down to the creek or the pond and haul up a bucket of water to flush it with or some other trade.
It may be years before the event happens. I suggest having your prepper sink, Faraday generator box and your outhouse on wheels. Roll the stuff out for testing and cleaning.