As with any electrical glitch in the Alpine, the first thing you ought to do is to turn off both battery disconnects: one for the Chassis and one for the House. One is in the battery compartment and the other is on your right as you go up the stairs into the coach (assuming a front door model). Turn them off for one minute and then back on. That resets lots of things.
If that doesn't help, another way of getting RPM info is from the "Diagnostic Port". This is located in front of the passenger seat and behind the kick panel which is velcro'ed in place. There is another in the battery compartment but this won't help for my next suggestion: You could think about buying a ScanGauge "D". The "D" is for "diesel". It plugs into your diagnostic port and reads many parameters. You get to choose 4 for it to read all the time and you can change which 4 anytime you want. Take a look at: ScanGauge - Trip Computer + Digitial Gauges + ScanTools
and click on the "D" model. Amazon can get you one in 1 or 2 days. On Amazon, search scangauged.
I own one and it plugged right in. I typically monitor water temp and fuel mileage info. It has one dual ended connector, one for 6 pins and one for 9 pins. It'll plug right into your 2004 Alpine with 400 Cummins.
I found that the cord wasn't long enough and bought an extension cord and a connector. It's a regular networking cable (Cat 6 ethernet cable-also available at Amazon).
That would be a quick and dirty fix to get you the RPM info uyntil you get home, and a neat gizmo to fiddle with later on also.