RPM gauge on dash does not match ECM

Hello, I'm having an idle issue with my truck. When driving, the truck runs fine, but recently when coming to a stop the RPMs will sometimes drop down as low as 100-200. This just started recently, and it does not do all the time, but will do it at some point every time I drive now.

Sometimes it will dip down gradually over 10-30 seconds, and sometimes it will dip down in quick jerks, maybe 200 rpm at a time.

It has not stalled yet, and considering that the gauge sometimes reads as lows as 100 rpm, you would think it would be running extremely rough, but it is not very shaky at all. Sometimes at the low rpm you can feel a slight shakiness, like a mild miss, but it is so slight that I don't think I would even notice it if I wasn't specifically paying attention to it, and it has yet to stall or even feel like it was about to stall.

I hooked up a scan tool with live data to take readings on the 02 sensors, but they seemed to be reading the correct voltage, jumping between about.2~.9 on the upstream sensors. When I introduced some propane gas into the intake, the voltage held at.8 or.9 consistently until I took away the propane.

As I was looking at the live data, I noticed that the RPM reading on the scan tool was showing about 650, while on the dash gauge it was only about 400 or so, and sometimes even lower than that. When I would press on the gas and bring the RPM up to 1,000 (according to the scan tool), I would look at the dash and it would only be around 600. This remained the case when I would go up to 2,000 rpm (according to the scan tool), but on the dash gauge it would only read as 1,500 or so.