sign-ups, billing, maintenance, plowing, clean-up for those who default, etc.

All of which the owner of a residential apartment building might be doing also, and reporting rental income on Schedule E.  The key fact here that has been omitted is, how many units are there?  The office complex where I rent has several dozen single-story buildings spread over many acres.  A full-time maintenance man and a part-time property manager.  It's still just another rental LLC.