Not sure what chassis voltage your coach uses, but the DDEC system operates on 12V only.
Somewhere near the front of the coach, the 12V taps off two battery sources (plus the ATEC).
If your engine harness is still intact, the DDEC leads are numbered.
There should be a DDEC B+ fuse that's hot all the time. I have no idea where to suggest you find the lead, however, if your coach is a 24V chassis, it's likely near the batteries. There will be a center lead from the batteries that powers the ATEC and DDEC ECMs.
There is also a relay near the engine that controls the 'ignition' source. DDEC ignition lead is 439. The B+ is 440...if the wires are OEM DD.
If your 'Check Engine Light' works, read the codes. If not, the unit isn't powered up.
If I recall, the engine room had an electrical fire?
Don't take the ATEC ECM apart...it's probably not causing problems.
You'll get a 'Check Transmission'/Do not Shift', if the DDEC isn't powered up.
Is the dash powered up? Have you traced the DDEC ignition and B+?
I'll bet that the DDEC is powered up, or being turned on by the ignition relay. That's where I'd put my money...for now.
Even with a fault sensor (low water, low oil), the engine would turn over and start. I would shut down within a few seconds and set a CEL indicating the offending sensor.
You may want to look at www.coachinfo.com
and verify that you have a DDEC II. If so, order a DDEC II diagnostic guide. I'd recommend not dinking too much with the DDEC system without this guide. The guide will enably you to easily figure out what's ailing the engine. Put 12V, or 24V on the wrong pin and you let the smoke out. That's very bad.
If your bus is a 12V unit, the DDEC fuses may be where the chassis fuses are located.
If it's 24V, look a the batteries and see if a lead, is tapped into the 12V terminal of the downstream battery.
If you have an equalizer, make that two 12V leads...? Some MCIs have 3 leads...for the headlight backup system.
Usually, the DDEC wiring and the chassis wiring are two separate systems. Only the ignition control lead interfaces with the chassis system....usually. There may be other systems that appear to interface, such as high idle, and cruise, but they don't stop the engine from running.
Keep in mind that the starter circuit is not part of the DDEC system. You may have had a fuel pressure switch in the engine room that has been disabled. If so, the starter won't work. As has been stated, it won't work if the neutral selector isn't indicating neutral.
Does your CEL function light up when the ignition is turned on? Both the engine and trans CELs should lite...so should the cruise control if you have cruise enabled. You have cruise but it may not be wired up.
Is your coach a 24V or 12V chassis?
Somethings to think about, JR