Remove chicken from package and remove any of the gizzards that might be in the cavity. Set the chicken on some paper towels to dry. Pat the outside of the chicken to dry it off too.
Slice the onion and place in the bottom of a 6-8 quart oval slow cooker.
Sprinkle the dry BBQ rub all over the outside of the chicken and rub it in good.
Don't forget to sprinkle some of the seasoning in the inside of the chicken too.
Place the chicken on top of the onions in the slow cooker. No other liquid needs to be added.
Cover and cook on low for about 8 hours.
Then, once chicken is finished. Pour half the bottle of BBQ sauce on the outside of the chicken and spread well.
Put the cover back on and let it set on low for another 15 minutes.
One packet of the dry rub is plenty for a whole chicken. I usually get two because I'm pretty messy with it and I want to make sure I have plenty to season the inside of the chicken with. Chicken will be VERY tender so take care when removing it from the slow cooker. I usually just pick the chicken straight off the chicken while it's still in the slow cooker and serve it up that way with a side of BBQ sauce. Serve chicken with the leftover BBQ sauce.