That's the easier part: a hole in the ground, possibly with a "thunder box" open air wooden box to sit on with a toilet seat, or an "outhouse" enclosure.

Water to wash with can be the harder part. Though having a dryish cabin, there's some convenient USB chargeable pumps that fit on a 5gal jug and little submersible USB chargeable pumps that get you a poorly pressurized shower nowadays from a big pot (which can be warmed on a woodstove, or sous vide thingy).

Great for the $28k-for-shelter and $20k-for-off-grid-land crowd, not so great for their hypothesized emergency relief scenario.

Compost toilets are the easy part.

A septic tank. This problem has been solved for literally thousands of years.

So I guess plan another $10,000-$20,000 for that.

Well, have fun shitting in a bag then. Perhaps you can also shower out a garbage bag too.