Some things you can do to reduce the workload of the units. 1) make sure there is enough insulation in the attic.

And I had insulation on them. 

There may be some tax benefits.As a quick fix have a home energy assessment done(should be free) and find out if a few hundred dollars of insulation could help keep heat in.My .02 here!

They did, though, renovate massively - new kitchen, new floors, new wiring (got rid of aluminum), insulation, etc.

New plumbing, electrical, roof, insulation, the works all done correctly with quality materials.

The roof have no insulation.

The idea of running a little water to keep things from freezing in the cold is common.It seems that practice was a contributing factor in this mess: https://amp.bangordailynews.com/2018/01/31/news/midcoast/maine-mobile-home-park-residents-weigh-lawsuit-over-water-problems/I'd look into a thermostatically controlled heat tape system on all the under trailer supply lines, along with insulating the lines.

There is a bit of aluminum capping involved, caulking and insulating, but other than that, the materials are under $200.

Does it have rough electrical, Top Out Plumbing, Rough HVAC, and insulation?

If you go the real estate route, look for property types that are better insulated to economic downturns and diversify (property type and geographically if possible).