If you are not concerned about Health Dept. approval you can make a small septic system with a 55 gal drum, 20 foot of perforated 4" pipe and gravel. I built one several years ago. Let me know if you need details.

As for the toilet/water system, use one from an RV. They run off a 12V battery that can be hooked up to a solar charger. Or better yet put the outhouse below the dam and use gravity to flush.

I'm 2-4 years off of doing something similar at our pond.