I understand what you want to do. It might be a little bit of overkill, but I can understand the appeal of easy troubleshooting.
- First use one of these http://www.wiringproducts.com/contents/en-us/p525.html
and connect the input to 12 volts fused line (I would use about a 50 amp fuse and holder) from a small battery home alarm battery, or motorcycle battery). This will give you six 12 volt fused lines.
- Run a wire from each output terminal on the fuse panel up to a toggle switch, so you can turn the power on and off. Now you have 6 lines of fused, switched 12 volts.
-Then install a 8 line terminal strip like this http://www.wiringproducts.com/contents/en-us/p893.html
and use a short wire to connect six of the terminals to the lines coming from each toggle switch.
- Skip one terminal (leave it open), and use the last one to run a ground wire to the battery ground.
Now you have a terminal strip with a set of six fused and switched outputs, and a ground point. Use that terminal strip to connect wires running to each type of trailer connector you want to simulate. You can connect any combination of trailer connectors, and use the toggle switches to put voltage on any of the lines. You could mount the trailer connectors on the side of the test box, or just leave them with short pigtails that run out of the test box.
I would not worry about measuring amperage and voltage unless you think its a problem. You could put an amp gauge inline with any of the lines between toggle switch and the terminal strip, but they would be full-flow amp gauges. I don't think its worth the trouble. You could put one 12 volt meter between the two battery terminals and measure the overall voltage. If you need to see the voltage to any specific output, just measure between the ground on the terminal strip and the output terminal.