We've taken two road trips that have required us to charge from the network of Tesla Superchargers and a 110V outlet in an Airbnb.
Tesla Superchargers in N America
Supercharging is a bit cheaper than gas right now, but not by much. (The biggest savings come from charging at home or an Airbnb.)
Unlike "filling up" at a gas station, when charging from a Supercharger, it often makes sense to charge only enough to get to your next stop comfortably, because charging slows as your battery fills up as explained below.
More detailed info regarding charging can be viewed about each of our road trips.