Just a thought ...
A friend of mine had a sewer line blockage on his city lot. Turned out to be roots from a tree invading the pipe. The plumber used a concentrated sodium hydroxide solution to 'burn' the roots out. Later I saw the same stuff for sale at Home Depot (it was labelled for that exact purpose).

Regarding outhouses/potty boxes:
I can vouch for sawdust from personal experience. I've been using dry sawdust from a truss manufacturer as kitty litter and down my occasional-use outhouse for two years. I used to use dry, cold wood ash; but nothing beats sawdust for moisture and odour absorption. It works astonishingly well ... better than commercial kitty litter ... though the cat tracks it a few feet ... it helps if I mix in a little dry black dirt. I'm going to try it in a camper, in a five-gallon pail, for 'liquids only' one of these days, as a test. The result is compost; I'll just put it on my trees (I live in the country).

(Aside: I plan to add some vertical ventilation to the outhouse as well - moving it toward a homemade composting toilet, the essence of which is removal/absorption of liquids and constant positive ventilation.)

Hope this trying situation resolves itself okay.