Howdy @Nick FerroniYou should have the septic system inspected (camera). 

I would not just put up a sign saying it is under surveillance, but actually install surveillance.

Just a few hundred dollars would do.Buy business cards, bandit signs, enough for a small direct mail campaign, some nice looking clothes to make you look trustworthy if you don't have them.Cheap digital camera off of ebay/craigslist.

If your friend is looking for a self storage unit that's reliable and secure, besides the obvious things like security cameras, I would advise him to look for two things in a facility:a.

I also can not find a company that can do an inspection via camera.

As for the homeless population, install bright lighting and cameras if possible.

The pool contractor could use a camera to view below the cover for example.

I'm not sure the price for that.  4x the sewer camera scoping if you are going to do that @ $100-200 a piece. 

I'm a remote property owner; I've got property management in place.I've had some crime (break-ins etc) at this remote location recently so I'm looking to install some cameras that I can view remotely. 

Have a plumber run a camera on each building's sewer line and give you a video.