I think you can do this without detecting the state of the TV, from either SmartThings using some automation like detecting no motion and triggering a scene that turns all devices off. Or by asking Alexa to turn your downstairs off. You mention the Harmony hub, so assuming you have this, I have the Harmony Elite though not sure if it makes a difference.
Setup two or three integrations dependant on the devices you have and what SmartHome device they connect to.
Alexa and Harmony from the following https://support.myharmony.com/en-gb/harmony-experience-with-amazon-alexa#whatsnew
Ensure that all the Harmony Activities are discovered and selected in Alexa, along with any lights or other devices it finds
SmartThings and Harmony from the following https://support.smartthings.com/hc/en-gb/articles/205653834-Logitech-Harmony-Home-Hub
This will only pull through your Harmony Activities but it allows you to turn on and off those activities from within SmartThings
SmartThings to Alexa from the following https://support.smartthings.com/hc/en-gb/articles/205275404-How-to-connect-Amazon-Alexa-with-SmartThings
You can be selective and oly select to synch those things/devices that are not natively supported to connect to Alexa
Once you have run Alexa discovery after all of those integrations, create a Smart Home Group giving it an easy Name to remember in the Alexa App. Select the Devices (Direct Alexa ones or Smart Things Integrated ones) and Harmony Activities that you want to turn off.
You can then ask Alexa to turn the Living Room off and it will turn off all the devices, using either Harmony, SmarThings or Alexa integrated. It doesnt care whether the device or activity is off or not it just issues the off command.
You can also Create a Scene in the SmartThings App select all Activities/Devices and have them power off. Though for this to be able to turn your TV off you need to have switched it on with a Harmony Activity from either SmartThings, Harony or Alexa telling Harmony.
You do have to be careful on what you Synch as you can end up with the same device visible multiple times via the different activities.