Fire box/safety box/emergency papers. What do you carry?

I don't carry one but, it's a good idea.

I would probably keep the registration and insurance card in the box. You'll need them in the event of a major fire.

And, a list of phone numbers in the event your phone doesn't make it out with you.

A combination lock is a good choice. But one option is to duct tape a spare key to the bottom of the box. If someone breaks into your RV they will take the box and break it open later.
The combination lock will not deter them from stealing the box.
JMO