Has Anyone Added a Deadbolt to the Bottom of the Door?

The dealer was right; they leave RVs unlocked to prevent damage "when" thieves show up. I've heard the same from several other dealers as well.

A deadbolt at floor level would be a nightmare to lock/unlock from inside. If it's use would only be for storage (locked/unlocked from outside only), fine, but again, it won't stop a thief that wants in, it will only increase the amount of damage to be repaired.

I would be more concerned about the (lack of) quality of the door itself.