Prepare the slow cooker by coating it lightly with olive oil.
In a small bowl, combine the brown sugar, paprika, salt, black pepper, and garlic powder.
Rub the seasoning mixture evenly over the pork tenderloin, coating all sides.
Place the sliced onion and garlic cloves in the bottom of the slow cooker.
Set the seasoned pork tenderloin on top of the onions and garlic.
Pour the chicken broth around the pork.
Cover and cook on low for 6–7 hours or on high for 3–4 hours, until the pork is very tender.
Remove the pork from the slow cooker and shred it using two forks.
Return the shredded pork to the slow cooker and stir in the BBQ sauce.
Let the pork rest on warm for 10 minutes before serving so it absorbs the sauce.