My opinion: FIRST if your NEW Ram sits out in that cold 24/7? - I would run a 5W-40 viscosity oil.
As for what temp to actually plug it in?
My 2017 sits in a warm garage (40-50°F) while at home during the winter.
I say to the **** owner like myself - if you have plans on keeping your New Ram many years past pay off - Plug it in (Any time ambient temps are 20°F & below).
My question on this matter is: How long does it actually need to be plugged in for - to aid in your next start up attempt? 3-4 hrs before hand?
Work - just installed (6 new plug in outlets) in the parking lot for us diesel owners to plug into. During 12 hour shifts we see a few different weather changes.
So I am going to take advantage of it!
I just bought a new cord & timer as well as a Noco Genius GCP1 adapter plug for the front bumper - for easy fast plug in.
Putting everything together now as we speak - will install everything this weekend with my winter oil change (Filter removed gives me easy access to the heater element. Also bought a 15Ft 15amp 14 gauge extension cord to keep in the back floorboard.
Dam Rambling Once again!
Think about running 5W-40 viscosity oil & plug her in @ any temp below freezing for at least 3-4 hrs before starting.
Maybe if much colder & if you don't care about the timer option leave her plugged in the whole time while parked.
Last question: I honestly don't think your doing damage to your 6.7L by not plugging it in.
But think of all those other smaller/weaker parts that have to work twice as hard turning that big bastage over @ 20°F or colder - Batteries / Starter just as a small example.
Also: Plugging your 6.7L in will mostly help in warm up for you inside the cab a bit faster & aid in the windshield defroster. (Though this is not my main concern)!
I have worked in a Plant environment for over 20yrs & believe me the cold & wind chill effects EVERYTHING - Mechanical & Electrical in some way - Heat is your friend! It does lengthen the service life of most equipment during these brutal cold times!
Timer automatically activates when temps drop below freezing!
Truth is - you might be better off getting an answer from a owner who don't care as much & doesn't over think every mod like I do!