Can someone tell me what this is?

Circuit breaker it looks

 

and the corroded part is the mounting tab.

 

I highly doubt it's a solid state circuit breaker.

It appears to be a ground and that block looks like a filter to keep "noise" off the ground.

Whatever you do re terminate that sucker and keep dielectric grease on it. It doesn't matter what it is as much as it's corroded and disconnected and shouldn't be.

 

It is something that you need to go to the dealer to get fixed.

 

Looks like a low voltage disconnect but if it is it must have been removed on the other end at the battery bank. Because if it was still in the circuit your truck wouldn't start in that condition at all.

 

It’s a circuit breaker

 

Looks like one of those ‘power cut-off’ switches that you flip off at the end of the day. Most likely the switch is on top, knob missing. Popular years ago, way before today’s electronics gizmo’s that require 24hr juice.

 

Yep, reading this post is fun though. What I can't figure out is whether it's a vanilla or a chocolate one.

OP needs to start the truck up and lick both posts as well as the end of that loose wire and tell us if it's vanilla or chocolate.