There is a lot more to the emissions system than the gas cap.
The cap is just the most common issue, and generally that is when the cap is damaged or the wrong cap.
But, since the code is issued
on a long trip sounds like it may just happen when there is more heat than usual... That will make it harder to find, because under "normal" circumstances, there isn't anything wrong.
You most likely have a hose or connection leak somewhere that has enough integrity to maintain pressure in normal circumstances, but loses pressure when subjected to more than normal, if indeed you have a good fitting cap...
You could examine all the hoses and connections between the fuel tank and the emission canister and examine the tank itself, and maybe you'll get lucky. Intermittent problems are hard to find, but you could pressurize your fuel tank and simulate the high pressure condition (put a hose tap on one of your gas caps and add a few pounds of pressure with a pump) and you might be able to hear the leak hiss. .
I would just clear the code and live with it, unless it happens more frequently.
You should also get a code reader that allows you to clear codes if you don't have one, so that you don't ignore a major CEL problem because the minor issue is hiding it.
That will also allow you to clear the code while on a trip, and see if the code comes back immediately or doesn't.
This information will be relevant to how the leak is leaking...