Here is my trailer light test box.
The battery came from an interruptible power supply for a computer. The flat four trailer connector was from an boat trailer I put disk brakes on, and the disk brakes require a back up light connection to disable the brakes, or you could not back the trailer.
I just unplug the spade connector from the battery, to test right turn, left turn, and tail/marker lights.
I also built another version of a test harness, for a boat dealer I used to work for, using a rotary switch. Later, somebody else added a turn signal flasher to it.
To be honest, you are proposing a really complicated solution to an easy problem.
You need two devices. A battery connected to a male socket, to take to the hitch end of the trailer, to check the trailer lights. Use a switch to each function up, or just unplug each lead like I do.
The other device is a hand held trailer simulator. A box with a female plug, and lead, you can plug into the tow vehicle.
Put small lights on it for each function, or a rotary switch, to hook each function individually to a single light.
Everything is 12 volts, isn't it? You do not really need to check the voltage. It is there, or it is not.
I would suggest you put some sort of two pin plug, that has a wire jumper, that you could unplug, and then hook an Ammeter to the two pin plug. A problem I see is that an Ammeter that could read the largest load, like all the marker and taillights on a 18 wheel trailer, would be reading a very small number if you are only testing a single LED light. leave the ammeter in the testing small load position, and you damage the ammeter, or blow its fuse.