Feel free to PM me if you have specific questions.
Obviously it is very niche but the upfront investment could be fairly low. There are a lot of complicating factors regarding doing it legally. Many of the 'houses' are actually travel trailers, and I think that to do it legally in numbers above 1 - you may need to create an RV park which could be rough. I think one of the real questions is how you handle plumbing if you were going to have more than 1-2. If you built legal houses, you would need to work with local zoning code which is accommodating to various degrees depending on where you are.