Great topic! Here are my photos from my 1979 31' International. Everything works though I've had to make some repairs.
Pardon me if I provide too much information but I've gotten PM's in the past asking for repair tips. While I am glad to help out when I can, these may be answers to questions you already have.
The gray tank level gauge always read empty when I bought the trailer in '04. I tracked the problem down to an integrated circuit (chip). There is one on the circuit board near each tank gauge. You can get one of these chips for only a few dollars at Radio Shack. The number is LM324N. They need to be soldered onto the circuit board, which is tricky. There is an indentation at one end of the chip to orient it the right way. Note that before removing the old one.
My control panel has been plagued with failing solder joints on the tank gauge LED's. If one of your LED's is out, retouching it with solder will probably fix it. If you have to replace an LED, it is possible to install it backwards, in which case it will not light up.
Note the photo of the control panel removed and hanging on the hundred or so wires. It is possible to work on the panel without unplugging all of those, just be very careful of that exposed circuit board behind the battery level indicator.
Once when reinstalling the panel, DC power to it was connected, I accidentally touched that circuit board to the metal enclosure. There was a shower of sparks, and I saw that several traces on the board were fried, and of course the panel was dead. I was able to repair it by painstakingly soldering some jumper wires to that board. You do not want to have to go through that. Be safe and pull the fuse before working on the panel.
My Univolt charger was long gone when I bought my trailer but I wanted the AC Power indicator on the control panel to light up when plugged into shore power.
Check out the photo of the fuse panel. Connecting a jumper wire between the first circuit on the left, Power On, and one of the Battery Negative (ground) terminals, turns on that indicator light. Now at first that didn't happen for me and I later found that LED to be burned out so it had to be replaced.
In order to make that jumper connection in the fuse panel permanent and automatic, I used a 12 volt
DC automotive relay from Radio Shack, number 275-226.
You will have to remove the frame and door from around your fuse panel to access the terminals but hooking up the wires is easy. Wires go from the fuse panel to the switch portion of the relay.
To make the light come on automatically when shore power is plugged in, the relay is energized by an AC adapter, which provides 12 volts DC to the coil side of the relay. Sorry, no part number for that AC adapter, it was something I had laying around.
I've attached a photo of the final install. My Progressive Dynamics converter/charger is in the foreground. Behind it is a 120 volt power outlet where the AC adapter is plugged in and secured with zip ties. The relay is secured to the left of that.
Although I have put a lot of trouble into the control panel over the years, I do like having the original equipment still working like new.