Bob,
The AIR FILTER MINDER GAUGE reads total restriction in the engine's air intake system, so indeed that means from side of coach opening, the (often too long) flexible hose to the air filter, hose/piping from air filter to turbo.
I would start by verifying that the flexible hose from side of coach to air cleaner is as short and straight as possible. I have inspected coaches where the chassis maker supplied hose that allows some flexibility in where the air intake is mounted is NOT shortened and routed straight. Cutting out 2' of flexible hose is a pretty easy and inexpensive "fix".
Some coach makers cut too small a hole in the side of the coach. That would be next on my check list.
After that, if you still have excessive restriction, see if you have room for a larger air filter housing with the same outlet location and size.