Okay, HOA aside, pork tenderloin doesn't like to be overcooked. If it was chewy, you most likely overcooked it. At that cooking temperature, the pork was probably done at twenty minutes.
If you want to stuff lean pork, use pork loin, not tenderloin. The extra meat will allow you to cook the stuffing through, without overcooking the pork. Always use a stuffing that can be safe to eat at 145 degrees, because that is the temperature at which your pork loin will be perfectly cooked.