That "little black box" is a 12VDC circuit breaker. There should be 2 or 3 located around that area, so look for the rest. They may be on the other side of that bulkhead or maybe even further away, like in the front passthrough or tucked away in the belly of the trailer... All the RV manufacturers "hide them in different locations"... Yours is "shot" and need replacement. First remove the canister and you'll likely find the amperage rating engraved, stamped or printed on the bottom of the container. The rating is located there as an attempt to preserve the markings from being scratched or washed away... Also, when you get the new one, you'll notice there are different color terminals. One is the load and one is the input. Compare what you currently have to make sure you install it properly...
You can buy a replacement breaker at almost any auto parts store, at every RV parts store and on Amazon. Just make sure you get the correct amperage so you don't end up with too frequent power interruptions and/or a circuit breaker that's rated more than the wires can handle, creating a fire hazard....