Mark, there are shops that rebuild speedometers. Yours appears to made by Spartan (according to info at the Beaver Coach Assist). But I would check the connections before doing anything with the speedo itself.
I don't know how your speedometer is connected but I would think there has to be receiving info from the transmission output sensor. I would check that area (transmission) to be sure there are not any loose or chaffing wires - then same at the back of the speedometer. I don't know that having the odometer working means the wiring/sensor is good and speedometer itself is therefore bad. If you contact a shop that rebuilds these, they may be able to give you some troubleshooting advice.
Googling the manufacture of your speedo didn't come up with anything - except some links to rebuilders and this link (maybe to purchase a used one)
Such as (this is example, I don't not know anything about this company)
https://www.instrumentclusters.com/The screenshot below came from the "96 Monterey Dwgs" parts list at the Coach Assist section but I'm not sure you can trust the info 100% so if you can get to back of the speedometer, try to verify the manufacture and model of speedometer.
http://beaveramb.org/coach-assist-members-only-new/Here is a link to a thread that might help if you need to replace your Speedometer
http://beaveramb.org/forum/index.php?topic=143.0Lastly, there are some RV salvage yards that may have what you need. Such as
https://rvcomponents.visonerv.com/cgi-bin/f/motorhomecluster-gauges.pl