Also, what are the physical requirements I need to have in-place? 

You can see here that while Tucson did recently make their ADU laws much friendly, Tiny Homes on Wheels do not qualify and a tiny home must have a foundation to meet the ADU requirement.

The tenants have been served the 90 day notice to sell (Jan 1) required in WA state.

If you are not a plumber or electrician then you do not require those tools.

If going with a full custom plan (e.g. you have a unique lot layout that will require a customized layout)… closer to $10K, including construction drawings, title 24 energy report & electrical layout, permit processing-Permits: $5K is typically what the total costs to the city will run for an ADU, even with the waived feesConstruction costs: $250/sqft + $10K-Building costs: typically $250/sf to start, for vertical building only.

Food trucks here are required to have a commissary (and show a lease) - many will never show up to the commissary, though.  

Without knowing any of the additional expenses this seems to meet the basic requirements.

Requires a little more work upfront like Travis mentioned but you'll get slightly higher rents and those tenants do tend to stay longer.

This means that they'll require more than the standard 10% down with a conventional vacation home loan.If you have any questions, always feel free to reach out directly. 

They could also run away with it– it’s not fun when that happens so be careful and compare what contractors in your area are requiring/asking for.