Step 1: Buy and install a new filter minder-- under $20.
If it reads the same high restriction, THEN look for the cause.
Be aware that the filter minder reads total restriction "up stream" of it's location.
So, too small an opening on the side of the coach, small diameter/flexible or too long a connection to filter, too small a filter or too convoluted a routing from filter to where the filter minder is plumbed are all CUMULATIVE.
Best advice for anyone-- install a new filter. RECORD THE AIR FILTER MINDER READING. That way you will know as the restriction builds and know when you need to change it.
With a new filter, if the filter minder reading is too high-- replace the filter minder (see above). If reading still shows restriction, FIX IT.