To get the most help here, the info you provide, needs to be pretty specific.
Like year make and model, pictures are worth a thousand words, and specific results desired, like how you intend to use the camper. (full time, wknds, boondocking, travel goals)
For example, it sounds like you want to restore what is missing, which is the simplest way to go, but if it's an older camper, what was standard then might not fit your needs, like adding an inverter or solar or LED lighting or TV and electronic upgrades. Think ahead to keep from re-doing things along the way.
Please describe the location and wiring going into the box you are trying to identify. Again, a picture is better.
Someone more able than I will be able to direct you to a source for manuals and diagrams.
Good luck, I look forward to seeing good results for you.