When you say you're using the Trip Planner, do you mean
Tesla's Web-accessible tool or the car's built-in navigation system?
When you use the car's built-in navigation, it already knows your current state of charge (SoC), so that's what it uses; it's assuming that you want to depart
immediately. AFAIK, Tesla's Web-based tool assumes you'll be departing with a full charge, but I'm not 100% positive of that. (I don't use that tool much.)
If either the Web tool or the car's built-in navigation system is showing you one Supercharger stop between your starting point and your destination, then that's all you need -- or at least, all that the tool thinks you'll need. It could be wrong because of temperature, speed, detours, etc. If such things happen, the car will update its Supercharging recommendations appropriately while en route.
If you want to plan the trip ahead of time, there are quite a few tools you might use. Two of them are:
- Tesla's route planner -- This is Tesla's official Web-accessible route planner, as already referenced. It's pretty basic, though.
- A Better Routeplanner -- This is a Web-based tool that offers a lot of options. Perhaps too many, especially for casual or first-time users. That gives it a lot of power, though; you can specify your starting SoC, weather conditions, what types of charging stations to use (if you've got adapters), etc.
Once you're on the road,
PlugShare can be quite helpful, too. It will help you locate non-Tesla charging for overnight stops or level 2 charging at your destination; or to help you route to a CHAdeMO or CCS station if you have the appropriate adapter and prefer to use one of those for whatever reason.
Note that there are iOS and Android versions of both A Better Routeplanner and PlugShare, too; you'll probably want to load them on your phone before you leave.