The small black tank looks like it came with the Air Force One braking system. Do you have that for your toad? I (think) the valve on the small tank is a pressure protection valve if you have the Air Force One braking system.
When you have a boost leak you will also have high exhaust temp. I would recommend anyone with a turbo diesel to have a pyrometer. With low boost you will see very high exhaust temp. Be careful.
That is an aftermarket over temperature protection device. If it sees the temperature where it’s connected on the tubes to get too hot, it will shut everything down. So either it has died and you need to reset it or you actually have a problem with the cooling portion of the refrigerator.