The problem I see is that you have no legal standing if he were to terminate the lease, sell the property tomorrow, or the relationship deteriorated and told you to go screw.

Granted you have older folks who move on as the neighborhood deteriorates  but the majority is tough to manage and handle assets. 

Even paint on cabinets only lasts 5 years or so before it starts to deteriorate.

Radiant Barriers would seem like a top frontrunner with Florida's heavy sun but wondering if it would be enough insulation and how much it would deteriorate the shingles on the roof.Spray Foam is far too expensive to have others install but is still pricey for DIY.  

There are long term expenses everybody glosses over like vacancy rates, tenants trashing the property, deteriorating neighborhoods, lesser appreciation in lower cost neighborhoods, etc.I wrote this to compare cash flow on a deal I did for an investor.

You can save them the cost of foreclosure, and imply that you are ready to walk and the house will start deteriorating

The biggest cost is doing Vinyl siding to cover up the 1980s wood siding that is deteriorating and is an ugly brown.

Modulars should not deteriorate any quicker with proper care. 

During this time - they are not collecting any payments and have to pay for the upkeep(real estate taxes and deterioration of property).

Deterioration, sinking, water?