Fridge filter is like a lightbulb it is a consumable that they can get or not get, makes the water better or I suppose the fridge smell less but it is not essential.   

I assume it has sewer/septic, water and electric and they 2 tiny homes are plumbed/wired?

If you have a 6 bedroom in an area that does not have a university (students looking for shared space) or is in a family friendly place it may sit vacant longer between tenants.For Capex, try not to use a generic number - look at the roof, furnace, hot water tank, kitchen - when will each need to be done and what will it cost?

Its extremely important when running tighter margins like what you're talking about to focus on age of HVAC, water heater, appliances, roof, etc.

Given this is where my major operating expenses fell (beyond taxes and insurance), keeping this cost lower, by average, helps my cash flow.I invest in older neighborhoods, so the 2-4 unit properties, typically have single water meter, maybe single electric, and many have radiator heat with a single boiler.  

In addition, I will also be paying much closer attention to water flow patterns and signs of erosion around our next investment!  

Still, if you’ve got someone on the ground who truly knows how to navigate these waters, it’s like being handed a map with all the back roads highlighted.

With the lower price point rentals, you basically just tread water.

That certificate would show open taxes, water, housing violations, citations, condemnations, receiverships, construction liens and other municipal liens.

An alternative to batteries is a "water battery".