For example, several of our clients last year did not have a primary intention of profitability. 

A lot of times the investment property is their former primary residence. 2.

@Robert StephensonIf you have enough equity in your primary or rental you can get a HELOC or HELOAN and use the proceeds as a down payment to buy another rental.

Even in HCOL areas, adding an ADU typically does not make sense because developing a single small unit is the most expensive residential development that exists.Adding an ADU is a lot of work (probably more than a brrrr but with worse return), has poor financing options, typically adds less value than the cost of the addition, and detracts something from the primary structure even if it is just yard and privacy.Good luck 

Again, to reiterate, the risk is less burdened because the property also serves as your primary residence.

Having a vision for future projects like the ones you mentioned are great, but learning the ABCs of RE with a small investment (initial down payment - since you only need 3.5%-5% down on a primary) is the best way to sharpen your skills.

Is this all handled by the PMS and then routed to the guest by whoever is the primary host?

I don't think I would be interested in renting a boat or RV....I would have it too long for that to make sense.Though I own several properties, I currently have no primary residence...  

In most cases, I don't recommend renting out your primary residence especially if you have no rental experience and that is because usually your primary residence was bought without the idea of renting it out.  

You cannot use those losses to offset your primary income unless you are a real estate professional.