I'm thinking the transmission in your 40' coach is at least an Allison 3000 series .
The trans would react to erratic speed sensor output with a code , that could be retrieved from the shift pad. PDF below .
For years the speedometers in vehicles have been electronic , and there is a very good possibility there are internal issues in the speedo head .
If you pay close attention to the odometer function , is the change of the miles readout accurate to the distance your covering ?
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99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.
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