You Allison trans uses at least 2 and on some models 3 speed sensors to determine proper trans operation. A poor connection at one of the sensors, sending a low or erratic signal to the controller can be to blame , or you've got the very first indications of a sensor failure. The sensors on my trans are on the drivers side of the trans , with a wiring harness connecting them , two wires in each connector ; my harness has 3 connectors but only 2 are used; sensors are 1 near the engine end for torque convertor RPM, and the other on the tail shaft, near the U joint.
Pull the connectors off the sensors, check for corrosion, use a spray electrical contact cleaner if any clean up is necessary. Follow the wiring harness back and look for any damage, and pull the main connector between the trans harness and the chassis, look for any corrosion there too, then do the same at the Allison trans controller, sorry I can't be sure where the control is on your coach , but it's about 8" square by 2" thick with the Allison logo on it , pull the connectors and check for corrosion.
Disconnect your chassis batteries before you start, any battery disconnect over 20 mins. should clear the fault code, re-connect batteries and road test.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.