What is accepted battery draw down?

It's not the batteries. You have a major ground disconnected.

 

Like I outlined in post #18 above.

 

Your dealing with very high currents be careful!!!!! Remove rings, chains and jewelry wear safety glasses around batteries.
Its not hard to diagnose, just test and eliminate one circuit at a time.
You need couple simple tools:
12 volt automotive test light (get a quality lite with long ground wire and good ground clamp).
Automotive jumper cables for checking grounds between cab, chassis and batteries.
Working multimeter.
1st. Trace all of battery cables from batteries to starter and to chassis. Be sure they are clean, tight and look good.

2nd.
Find ground cable between cab / sleeper and chassis or battery. Take apart, clean and inspect.
With engine off; Turn on loads to cause light problems.
Check chassis ground between cab and engine and or chassis with test lamp. If lamp lights you have found problem.
Use jumper cables to provide path to ground. If this fixes problem you will have to find open ground wires or fabricate new wires.

If not keep trouble shooting...
Somewhere in the cab and sleeper there will be common ground post(s) where all the ground wires are connected to a stud or buss bar.
Use test lamp between common ground and negative battery post. If test lamp is on, you have found problem, Use jumper cables to provide ground.
Re test.

Keep going, testing each subsystem until you find the problem.

The way you describe it's a ground problem. It is possible the problem is on positive side of wiring, same procedure's EXCEPT do not use jumpers!
Disconnect batteries, clean and check connections between fuse panel(s) and batteries.

 

You're working on a Cascadia, correct?

This TSB says there is a Mega Fuse Junction Box on the frame. That probably is in the circuit between the cutoff switch and the PNDB.

https://static.nhtsa.gov/odi/tsbs/2014/SB-10039612-4715.pdf


This TSB shows an updated PNDB for loose fuses. Did you install the updated version?

https://static.nhtsa.gov/odi/tsbs/2019/MC-10166800-0001.pdf

Check the fuse for the instrument cluster. Does it have power when the cluster fails to start up? Is there a relay powering that circuit that's possibly going bad?

 

I’m not familiar with a Cascadia but my advice would be to start looking where wires are bundled and tied to the frame. My FLD had issues and it was where they had stuff bundled where it passed thru a crossmember. My last 579 had issues and it was where they had wires bundled and cinched tight on a bracket that came off the frame. The factories tie stuff up way too tight and don’t leave any slack at all it seems.