As long as your soil will percolate, a septic system isn't difficult to build. You just dig a trench leading to a hole, fill the hole with plastic buckets or 55-gallon plastic drums cut up and arranged correctly, then run a correctly sloped PVC pipe to the septic tank you just built. From there, you connect to your fixtures to the drain pipe as needed.

The difficult part is getting the tools and equipment to do all that out to your location. The bigger the tools you use, the faster and easier the work will be, the it will also be more challenging to get it in and out. Basically, a shovel is a lot easier to carry than a backhoe, but a lot less pleasant to dig with. Same deal with a reciprocating saw, battery, charger, and solar setup, versus a hacksaw.