It has one STR penthouse (5 bdrm 4 bath with 3000 sf roof terrace),  plus 5 duplex units (Each duplex is two 3 bdrm 2 bath). 22 parking spaces.

Meanwhile that capital is not put to work, unless they park it in stocks, bonds, mutual funds, etc which are subject to market volatilities and shouldn't be used for shorter term investment horizons. 

There can be shared laundry onsite, shared parking, shared (yet separated) yard space etcAs for the math portion, I run my rental comps on rentometer and compare it to other ADU's or 1/1 units and their rent amount.

Are trailer park homes good investments?

I do mobile home parks.

Kids are adventuring and walking too and from school unattended, people park where they need to, people use common sense to make decisions more than relying solely on the rules, etc. 

It's a way better experience if you can find a local resource than these larger, national servicers.

Same if you park it (your cash, not your ride) and not use it at all. 

Of course, you can't protect yourself completely, but this will also help you prepare for buying the next properties a little better, and it's an exercise I generally do when I'm on a showing for a property I'm serious about so that I'm factoring this into my analysis to some degree in a "capex" reserve.Capital ExpenseRoofWater Heater x2All AppliancesDriveway/Parking LotBoiler x2FlooringPlumbingWindowsPaintCabinets/CountersStructure (foundation, framing)Components (garage door, etc.)

I've done roughly 100 flips since 1998, and routinely work with several local hard money lenders in my area as well as a few national ones.