Hi,
You mentioned that it was different with Key off versus key off and Isolation switch off. Check both the key switch and isolation as these may be worn and may be impacting your tests. As you are in the US, you contact the Combilift US based Technical Team. They are based in Greenboro NC. If you go to www.combilift.com, scroll to the very bottom, you can submit a "Technical Support" request. The form asks for truck SN, contact tel no, etc to help identify the problem and to contact you back. The tech team will then contact you. Here is a direct link here https://warranty.combilift.net/technical-support-form/
There is no limit on the questions, truck age etc, and no charge for the support

Hope this helps,
Richard