I'm assuming you're referring to charging the trailer's battery off of the car's alternator. I don't know for certain (never owned a Casita) but the trailer almost certainly has the needed connection already.
Most vehicles with 7-pin connectors as part of their tow package do require that a fuse be installed in order to activate the 12V pin. Information about that should be in the vehicle owner's manual.
If the hitch and 7-pin connector were added later by a third party, that party may or may not have run a 12V wire back to it. They're not required, so it's anyone's guess.
๐BTW - if it's planned to do this regularly while camping, this is an extremely inefficient way to charge the battery. Cars usually don't output a lot of amps to the trailer connector. You'd have to leave the car idling for hours. (It's good to have for the drive out/back though.) For regular use while camping, solar or a generator are better choices.
2021 Four Winds 26B on Chevy 4500