[MrE]

So I did some further internet sleuthing into your problem. I have additional questions;
1. Has VMSpec ever worked in your coach?
2. Did the coach ever run properly after the ECM was changed?

Your engine originally came with a DDEC III ECM and there are known issues replacing the DDEC III ECM with a DDEC IV ECM. GOING FROM A DDEC III TO DDEC IV ECM?

Here is a link to the DDEC IV APPLICATION AND INSTALLATION MANUAL on the WOG forum. There is way more information in the 749 page pdf than you'll ever need but it should give you the information needed to diagnose your DDEC diagnostic port you're using for your VMSpec.

This is an image of the connector guide from the Silverleaf website.