If you are showing that much vacuum in the air intake, I would say you have a problem. If not the filter, something else in the intake plumbing. How is the perfromance? Mine begins to fall off when the filter minder gets to around 50%.
A diesel is extremely dependent on a large air flow. If a new filter minder gives the same reading, you need to think seriously about enlarging the air intake and filter size. By the way, make sure it is the right filter. Maybe a previous owner stuck in a non-standard one that is simply too restrictive.
Former owner of 2004 American Tradition and several other RVs
Home is West Palm Beach, FL