Hi Clara,
The picture in your photo album helps greatly. A link to your album so others can see and add to the conversation.
https://www.sunlineclub.com/forums/members/14387-albums925.html
We still need a few more pics and questions/answers to try to create a complete answer for you on how to hook up the battery and sort out the electrical system.
Yes, you have the 7-wire plug; that is a good start. The plug needs to be cleaned with contact cleaner, and all the dirt is dug out ideally before you use it.
First, a few things to state. This black box is called the "Emergency Breakaway" switch. It is part of a federal law that trailers with brakes must have a device to immediately engage the brakes in case the trailer comes unhooked from the truck. There is supposed to be a cable from the pin on that switch, and when the pin gets pulled out during the decoupling of the trailer, the switch will send power to the brakes to stop the camper. The onboard camper battery supplies the electric power to engage the brakes.
Right now, that switch is bad, meaning it needs to be replaced when you want to tow the camper down the road. From the looks of it, it may be the original from 1982. Those switches are prone to water intrusion, which corrodes the inside of the switch, making it inoperable. They do not cost that much, but inflation now has them ranging from less than $10 to $40. And when you read the instructions, they list they should be replaced every 3 to 5 years. These are a very forgotten safety device. Here are two brands that I have used; both are as good as the other, and they may all come out of the same factory offshore.
Curt brand
https://www.amazon.com/CURT-52010-Trailer-Breakaway-Switch/dp/B001GN6QQW/ref=sr_1_6
The Bargmen brand.
https://www.ebay.com/itm/235938117550
You can buy those switches in many places. NAPA auto parts stores, RV dealers, or almost any place that sells RV parts.
If you live in a state that has annual required trailer inspections, they will fail the inspection if this switch is not working. While Ohio does not have this inspection law, a trooper will ticket you if they stop you for something if you do not have it. NY and PA, the last I knew, were trailer inspection states. PA was, for sure, 3 years ago; I had to help a friend get theirs inspected.
We need a good top picture looking down from above that shows the entire A-frame from the tow ball coupler to the front of the camper so I can see all the wires in one picture. I can then draw what should go where on your pic after we figure it out. I may have sorted out what they were doing with the wires, but I need to see them all at once to ensure I'm seeing them all correctly.
Next is where the seven wire plug wires go into the camper. This pic shows the black flex wire loom set of wires going up into the camper.
Follow that set of wires inside the camper. There has to be a junction box of sorts connecting those wires. There might be a small wood box with a cover, and they made some connections under the cover. That junction box is likely on the floor inside a cabinet or bunk, etc, straight up from where the hole is on the bottom side of the camper. There might be a fuse or two in that box.
We need to see pics of the wire connections clearly and any fuses in that junction box.
This picture is the old way Sunline installed fuses in the body and tail lights of the camper. They stopped doing this as the years went on, but the older campers have it. This all looks normal and intact.
For your power converter,
I need you to take a picture to show the wires on the right side in the red circle. It "looks" like a big thick black wire, and a big thick white wire may go into a red outer casing 2 wire cable. See here.
That thicker wire red cover cable may go right to the junction box. You will search for those junctions with the 7-wire truck plug wires. We need to confirm this.
This red cable coming back out from under the camper "might" be the same cable that shows up in the red circle at the power converter. See if you can trace that out and if it is indeed that same cable.
On your power converter, I cannot find anything on the web about that brand Converter Systems from Battle Creek, MI. They may have gone out of business years ago or been bought up by another company. If you have any manual or paperwork, that would be super helpful. It may or may not work. According to the description on the label, that is the older standard charge 13.65 volt DC power converter. It may always put out the standard charge, and it may need a battery to function correctly. Some of the older power converters worked that way. Some were known as battery killers as they never stopped charging and boiled out the battery if you left them plugged in and charging for days to weeks. More on this as we sort out all the wiring.
It looks like you have only one 15-amp circuit breaker. This is common among older campers;
I'm not sure if you have any wall outlets. Do you?
I can't see where the wires go from this picture.
Please take a picture back a little ways so I can see how many cables run in or out of this box.
I see this pic of yours; I'm not sure where in the camper it is, as we do not know the model number of your camper yet. I see the drain pipe, the hot and cold water line, an older black Romex 120 VAC cable, red single strands wires, and a red cover cable that they used on the 12-volt DC wire back then.
I know you say 1982 15 ft camper, but that is not the model number, and the prior owner may not have known it either. But we can help you sort this out.
In our FILES section, we have copies of the Sunline sales brochures with all the floor plans and model numbers next to them. You can download a copy, see which one lines up with your floor plan, and read the model number. Sunline calls out the camper length as the distance from the tow ball hitch to back bumper.
Have a look, and then tell us your model number. Maybe I can sort out that pic better.
I hope all this helps.
John