Are you reheating slices or a whole pork tenderloin?
If I'm reheating slices, I generally reheat in the microwave on level 6 in a covered container. I have also reheated slices on the stovetop in chicken broth or a new sauce that I'm serving the second time around.
If it's a whole pork tenderloin, I think I would bake it to reheat. I would wrap the tenderloin in foil or put it in a cooking bag at bake at 350 until heated through. I'm guessing this would take about 20 minutes or so. (FWIW,I have never done this before)