The only place they WILL NOT JV is in my home city of Philly... go figure. something about taking too long for permits and inspections.

The costs of land  , engineering , site work , permits , labor , and building materials make this virtually impossible . then there are zoning issues and the rest of the politics that go along with this . 

Let’s talk about your permit.

Some suggest investing out of state for better cash flow, but that's not an option for me at this time.Recently, I ran into a realtor who suggested mid-term rentals to construction workers as a more profitable investment model.

Nothing wrong with being an “essential” worker

The zoning will define the permitted uses (this will tell you if you can have an RV/Boat storage there) and it will define the density (this will tell you how many tiny homes you can add).You'll want to make sure you don't conflict the existing duplex to a new use or mixed use where you might put the existing use out of compliance. 

So yes timing turned out to be critical, as now those permits are like pinky rings.

Nor is there a permit to do so. 

Check your local (city) municode to see applicable zoning rules/permitted uses.

Buy a property with lots of bedrooms and rent out the spares to family, co-workers, friends?