why my motor will not turn off from the key

If it shuts off within a few minutes after the key is removed I would lean towards coolant leak. Due to the diesels high compression ratio and combustion temp the leaking coolant turns the engine into a steam engine. Check yohr coolant level.Google runaway diesel engine scary stuff. That's just a possibility though.

I am not disagreeing with John, his answer is good. I just have another possibility. The diesel engine has no ignition system, when you shut off the key what happens it sends a signal that shuts off fuel to injectors, killing the engine. The relay to the shut off may be bad. Cummings? Detroit Diesel? Cat? what?

If truck is equipped with a Qualcom or aftermarket radio check the wiring harness. I found a short between the Battery and Ignition wires. Once I repaired the chafed wires all is good.

A bad fues pannel its back feeding power to the ing fuse

Where r all cut off switch on 1999 airstreams Freightliner motorhome cutter