Thanks to all who have lent their advice on my original thread posting. But to clarify, my question was two-fold. First was who leaves a Berkshire/Charleston engine brake on all the time. I now realize that there seems to be multiple forms of engine braking. It appears that the one I'm referring to is a retarding of the turbocharger. Reading the many responses and studying further the manuals in the rig, I now understand that certain conditions (wet,snow,etc) should dictate that the engine brake should be shut off.
Second, and the real reason I posted the question, was gas (diesel) MPG. Again, leaving it on all the time, I have not broken the 10mpg barrier but once in the 4k miles I've driven the unit and that was not for long. Some of you have mentioned 11mpg and even as much as 13.5mpg (7.2 L Mercedes). I have also read that the Cummins isn't as efficient, but I'm not sure of that comment. My rig is a Cummins 360 hp.
So, the real reason for the post was the MPG question. Comments relating to anyone's experience would be appreciated.