Looking for some guidance

It sounds to me, that you know what the problem is. It's your slug thingy.
Seeing as no one else has one like it (made it yourself with some "additional" circuitry), no one will be able to help you really.
Only thing we can suggest is taking off the slug thing.
If everything else is perfect, and it only does it when the gizmo is hooked up, it seems pretty obvious to me.
No way of telling the Jake brake circuit the true elevation I guess?
Martin

 

You can't afford to run at 4 to 4 1/2 mpg either for what it will save you. You have a 6.5 to 7 mpg truck. Something isn't right in there.

Start with the intake boots and go from there.

 

My mistake, I thought you said, you took the slug out, you get crap mileage, you put the slug in, you get crap performance on the jake and cruise.

You can't have both worlds when you fool the ECM like that. It sees parameters and it has specific curve tables that it expects to be able to adjust to.

Not something you can just play with unless you have all the software and computer hook-ups.