Line an 8-quart crockpot insert with a slow cooker liner. Spray generously with non-stick cooking spray.
Using a mixer, cream margarine and sugar. Add eggs and blend until incorporated.
In a separate bowl, mix flour, salt, baking powder, and baking soda. Add to wet mixture, and blend until just incorporated.
Add whole bananas to the batter, and mix on low until mashed.
Add cake mix and oil to the batter, and mix until fully incorporated.
Pour batter into prepared and lined crock pot insert. Spread the batter so that the center has a more shallow layer than the edges. Cover crock pot opening with a kitchen towel or several layers of paper towels. Seal with the glass crock pot lid.
Cook on low for 3 1/2 to 4 hours or until the center is no longer wet to the touch. It is normal for the center to appear less cooked than the edges.
Remove crockpot insert from the pot, and let sit on a cooling rack for 30 minutes.
Remove the slow cooker liner from the insert. Gently remove the bread from the liner.