I swear I read an ingame hint that said Workshops would share my stuff between them, but that doesn't seem to be the case. I have a settlement at Sanctuary and Red Rocket Truck Stop, but I can't access my items I added in Sanctuary at RRTS. Am I mistaken in remembering that you could do this, or is there something I need to do to get that to work?
Yes you can - in order to do so you must the Local Leader perk at level 1 and have a spare settler and use that settler to set up a supply line. Enter Workshop mode and select the spare settler and press the highlighted button for Supply Line:
This provides you with a list of your eligible settlements to set up a supply line with:
All supply lines are visible from the map: