While I loved Bruce's idea, I have a hard time just doing things simple. Also, this is my second school bus conversion. When I did the first one I had a combination/key lock then also. It was a manual combination lock. The main reason I opted for that type of lock was if I were camping and wanted to go swimming or just didn't want to have to take keys with me I could just dial in the combination and open the door. Nothing like losing your keys at the beach or the pool and can't get back into the house/camper.
Step 1: Gather Supplies And Get Started
Things you'll need:
Keyless digital lock (The lock I used.)
Heavy duty slide bolt
Scrap pieces of wood (or plywood)
Extension lock piece
Various power tools:
(and outer stuff depending on what problems you run into)
LETS GET STARTED!
If you have a door as this you'll need to remove the white safety bar.