2006 Western Star electrical mystery

First rule of troubleshooting. Check the last thing that you did.
Given the age of the truck. When you changed the batteries you disturbed a wire that was getting flaky. Seeing as the gauges are wonky, I'd suspect a ground wire.
Look at the battery cables. Look for one that may have insulation bulging, corrosion inside . I had a corroded washer on a ground stud on frame. It allowed the connection to get loose.
Wire may have a break in it just beyond a crimp connector.
It'll be something simple when you find it.

 

I too would go back to the last thing you did and start checking wires and cables.

Being intermittent you might have a cable that's been rubbed bare in one spot that you can't see if you don't look for it that makes contact from time to time and shorts out.

Seen it a number of times, also airlines that were rubbed through.

 

I have the same problem 06 western star truck dies losses voltage I can cycle key a few times volts come back truck starts may run 1000 miles or 2 miles had 4 shops looks at it changed batteries cleaned ecm plug cleaned bus bar on firewall changed 70 amp ignition relay changed ignition switch !! It’s still not fixed ... what did u find out with ur truck ? Any help is appreciated .. thx

 

I had an '06 with the MBE4000 and it did weird things with the gauges all the time until I changed the cab ECU. It's the silver box behind the tach cab side of the firewall. There's an updated unit that has a harness mating two plugs to one which reduces 3 to 2 as I recall. Anyway the dash gauges intermittent refusal to respond and done other quirks ceased.