I was told that by setting the parking brake each time the truck is parked, the shoe-brakes would self adjust. Is this true? There are no access holes on the back plate for manual adjustment.
Not sure on your particular vehicle, but most auto-adjusting drum brakes work when applying the foot brake (not parking brake) while the vehicle is in reverse, so whenever you back out of a parking space and hit the foot brake, the brakes adjust.
There is a device within the braking assembly which looks like this:
There is a lever arm attached to the brake shoes, which when in reverse and the brake shoes move due to the stopping action, turns the small studded dial on this device which spreads apart on a screw action, causing the brake shoes to move further apart and adjust to fit. It will only adjust this as far as the shoes can moves (meaning there is a correlation between movement and the amount of adjustment), so over adjustment is not possible.
While you may have looked for a slot to adjust, it should be there at the bottom of the backing plate. It may have a rubber plug in place to keep debris from entering.