Place the chuck roast in the slow cooker insert. Sprinkle the ranch seasoning mix evenly over the top surface, then sprinkle the au jus gravy mix on top. Do not add liquid.
Lay the four butter slices across the top of the roast, spaced evenly. Scatter the pepperoncini peppers around and on top. Pour the brine over if using.
Place the lid on the slow cooker. Cook on LOW for 8 to 10 hours or HIGH for 4 to 5 hours, until the internal temperature reaches 90 to 95°C (195 to 205°F) and the beef pulls apart easily with a fork.
Turn off the slow cooker. Leave the lid on and rest the roast for 10 minutes.
Remove the roast to a cutting board. Shred with two forks, discarding large pieces of solid fat.
Return the shredded beef to the slow cooker and stir into the drippings until fully coated. Taste and adjust salt if needed before serving.