I think I can build all of that out using portable sheds as shells for about $75,000 and then if they rent for $100 per night only 8 nights a month that would still gross $4,000 a month ($48,000/year) and the ROI on that would be better than 5% per month.

I would look for a less expensive way to solve the problem such as adding window, portable, through-the-wall, or mini-split AC unit in the area you’re having trouble keeping cool, if possible.

Another option is a few multi zoned split units.we opted for a portable AC in our rental unit, as it was $400, rather than the $6k they wanted for a AC system mated to the furnace.

For future reference, any vacant property should be checked on daily and ideally have a portable security system installed inside the property.

It was such a pain to pull off to the side of the road, walk to the nearest gas station, spend a small fortune on a portable gas can, hike a bunch of miles up and down the mountain, spill half of that gas trying to get it into the tank because those spouts are designed so poorly, and on and on…Even though quite a few people have experienced this frustration one time or another, it is a relatively rare occurrence.

I've tried a portable unit but it can't keep up on hotter days and has to run 24/7 and is overall inefficient.

They said they would prefer to get a portable unit and  Install it in the living room where it tends to be the hottest.

That will put the fear of the financial gods in the tenant because Sec 8 DOES have the power to revoke the tenant's portable voucher.Let the tenant know that they must adhere to the lease, including housekeeping (local health codes), and allowed the number of tenants/residents in your unit. 

We keep a few portable air conditioners on hand to place at properties if they have no A/C at all and it will be longer than 48 hours before we can get someone out. 

I'd buy a portable generator (tax deduction) and keep it in the garage along with a 5 gallon tank of gas with Stabil.