Grease the slow cooker pot with cooking spray or grease.
Mix together the ground beef, oatmeal, egg, milk, thyme, onion, and salt & pepper to taste.
Take two sheets of aluminum foil and place them on top of each other in opposite directions. Place in half of the slow cooker. That will be your meatloaf half.
Place the meatloaf in the foil area in the slow cooker.
Clean the potatoes and place in the other half of the slow cooker. Try to place the potatoes as loosely as possible.
Set the slow cooker on the lowest setting. I used the warm setting on mine because low is too high for that length of time. Cook on warm for 10-12 hours. 7-8 hours on low, 5-6 hours on high.
Make the sauce by mixing together the ketchup, brown sugar, and the mustard. Set aside.
About 1 to 1½ hours before you plan to eat, put the ketchup sauce mix on top of the meatloaf. Add the butter and all-purpose seasoning to the potatoes.
Once the potatoes are fork tender and the meatloaf is no longer pink inside you are ready to eat.