The gas bill is very high and the apartment we viewed had the thermostat set at 75 degrees while the occupant was at work!  

I was always tempted to drop in first to see if it was an easy fix before hiring a pro because a heating contractor visit to push the mode button on a thermostat costs just as much as one to fix a real issue.I also subscribe to the theory that I need to be able to frequently see what I own.  

Each apartment has a pop top valve with a thermostat.

Yes you should be fine as long as you keep the thermostat set at reasonable settings (check local state ordinances, some have specific min temps).

The winters here are freezing - how do the landlords out there manage their thermostat/heating bills? 

Very few Class A communities include a smart home package with locks, thermostats, and garage door openers.

Federal regulations have been written that give consistent and concrete direction to home and commercial builders on construction of accessible buildings and units.The seven basic requirements of fair and accessible housing are:An accessible building entrance on an accessible route.Accessible public and common use areas.Usable doors (usable by a person in a wheelchair).Accessible route into and through the dwelling unit.Light switches, electrical outlets, thermostats and other environmental controls in accessible locations.Reinforced walls in bathrooms for later installation of grab bars.Usable kitchens and bathrooms.- See more at: http://autismnow.org/articles/reasonable-accommodations-and-accessibility-in-housing/#sthash.eHAZrjM4.dpuf

I don't know what kind of heater this is but electric heat is expensive in our area and poorly functioning units can sometimes draw more energy, if the thermostat on a unit doesn't work or the heater doesn't sense temperature correctly you can have a situation where it is always on drawing electric. 

Both have submetered electric and one has individual thermostat controlled electric heaters in each unit.The major expenses that the landlord pays for between the two properties include:Common oil boiler (10 yrs old) feeding steam system ($9k/year oil bill) in one property - does anyone have experience with gas conversion/replacement and submetering with a steam system?  

If so, you might take the thermostats, heating elements, and drain valve off of one of the old heaters, and put them in your stash.