My front door is a two piece air operated that opens outward, I've heard them referred to as a 'clamshell' door i think. I put an eyebolt in each half near where they meet in the center (where the door opens) and use a bicycle lock.
My bus is a rear engine, so has no back door (that's practically useable), but on the side door I used a standard house type deadbolt. The door is thinner than a house door, but the distance from the outside face of the door to the inside of the jam is exactly 2-1/4", the door thickness that the lock was made for, so just mounted the deadbolt with a piece of 1x4 under it, made it an exact fit. The lock was 3-1/2" high so no trimming the width of the 1x4 either. Have to cut a 1-1/4" hole through the door to install the outside key part. I didn't install that part, since the bottom of the door is too far off the ground to get into anyways, I just use the front door, but it does look neater on the inside than the long bar and latch that came with the bus.
Oh, you also have to take the lock apart and reverse the bolt, since the door opens out, rather than in as a regular house door.
Seems like you'd be able to use one of these on the front door too, you'd have to cut a slot in the step for the bolt.