How about a test light and spare fuse kit? A simple 5 amp fuse can stop you from being able to go onto the next camping spot.
Keep in mind though that it sometimes it can be more than just replacing the fuse (there may be another cause of the blown fuse).
I go on many service calls in Washington state to a truck weigh station where they have been pulling motorhomes over for burned out light bulbs, and they won't let them leave until it is fixed, so light bulbs are a must. Kind of strange cause most states issue fix it tickets.