Selinoid

I’m try to help a friend work on his. The big boy in the rear run box stays hot all time to touch and the ignition selonond beside it keeps going bad

A DP doesn’t have a neutral safety switch...will start without air in the system and with the parking brake on or off. I do not know of a diesel starter that doesn’t have the solenoid mounted on it.
I did look in my front run box and there is not a fuse for the starter there. I previously posted that there is a fuse in the rear run box on the board that lets you start from there.

In other posts, OP talks about a starter solenoid. This will depend on what motor the Monaco is equipped with. He also says an 08 Signature and does not specify engine.
Some have the starter solenoid mounted directly on starter.
Some rigs have a starter relay in compartment under driver's area on the exterior of the rig.
I would add that there should be a neutral safety switch and possibly one connected to the parking brake system.

There are more than one type of solenoids. What are you working on? The leveling or?