where can I do this (purchase a lot and put an rv on it)?

It's not a permanent residence, so trying to apply building codes might not be the right approach, folks...

Look for unincorporated land where there isn't a HOA or other county/town rule which says you can't park/store a vehicle long term.

As long as you're not dumping black tank waste on the ground, I don't see why your holding tanks wouldn't suffice. Solar panels on the roof go a long way towards not needing electricity, and you can always bring water to fill your fresh tank.