Pretty handy.... I already am using one of those 7-way testers that plugs into the truck side and has small led's to show what the truck is doing... Always lets me know if the I am having wiring problems on the truck side or the trailer side...

I have all of my 7-way connections on the trailer side in an electrical box mounted on the inside of the trailer frame and was always going to run a couple of wires from the 12V and Trailer Running lights to a wall switch mounted on the wall near the entrance door to turn on my running lights for emergencies to let Rescue Squad or Police know which trailer I was in at the campground when needed...
Your box would be very handy to trace down trailer safety lights problems...
Roy Ken