engine won't start

Appreciate feedback and am looking into all the suggestions and most have been checked out.

If it cranks and won't fire, I would suspect crank sensor, or cam sensor. A good scan tool will give you the answer

Another vote for cleaning the ground connection where the negative battery cable and a ton of other stuff grounds. It has fixed us 100% of the time with the same problem.

My book shows e-13 pin of the ECM being fed through a TCC/brake switch.
Which ECM fuse has no power? Some are fed through the ignition switch.
Have you tried moving the tilt steering wheel when holding the ignition switch in the start position?
Did he check the small circuit breaker in the fuse panel under the dash. Upper left corner of fuse block?
Did he check to see if fuse link by starter is good?

start at the battery, replace it with something, clean ALL PARTS of the connection and remove the FRAME GROUND and clean, then see what you got. easy first hard stuff next.

When did problem occur?
Is it possible you have an Interlock system(s) that is disabling Ignition power?

If you have no power to one ECM fuse, why would he think the ECM is bad? I would try to find out why there is NO power going to the ECM.
This could be a dirty connection, a broken wire, or a rodent chewed wiring harness. You have power to the ignition switch, other wise it would not crank. I believe coaches of that vintage seemed to have a problem with the ignition switch going bad and burning up internally because of so much power being drawn through it. Check all of the power going in to and out of the ignition switch as well as the connectors coming out of the steering column.

If there is no power to ecm then it is not ecm sounds like you have a bad connection or a bad main fuse somewhere. Power missing to all thise things is generally NOT the ecm