30K is the standard interval for changing the spark plugs on R/T's. SRT spark plugs are changed at longer intervals as far as I know.Why do spark plugs need to be changed at 30k? I thought most modern cars were good for 100k miles before something like that needed to be changed? Same goes for changing differential fluid before 100k miles. Rule of thumb from most mechanics I know some of which worked as Nascar crew chiefs. Say the risk of changing before then gives the risk of putting foreign objects into the mix causing the part to fail prematurely. Unless you have a reason to fix it the old adage if it ain't broke don't mess with it should apply outside of general oil changes and basic maintenance. A lot of the dealer subscribed maintenance are just money grabs common sense should weight in here.
On the sparkplugs, check your manual first.
My 2014 is 100,000 mile interval. I don't know when that changed on the hemi, my truck is 30,000 interval.