On my Coach the transmission temperature "dash gauge" has its own sender unit mounted on the transmission cooler hose at the rear of transmission. You doint say what model/make Coach you have, but for a 2003, I think your coach would be the same. You can test the wiring and gauge by disconnecting the temp sensor wire at the transmission and with the wire disconnected the gauge should peg out(cold) - then ground the wire and the gauge should peg out in the other direction (full hot). If it does that then the wiring and gauge are okay and the sensor is bad.
I had to replace my sender unit once already. But I consider the dash gauge jut to be ornamental. I don't rely on it for accurate information.
The trans temperature that the computer sees is from an internal sensor in the transmission and the most accurate way to monitor temps.
If you don't have engine/trans data monitoring (like with a Silverleaf or Aladdin or such) you can add a "Scan Gauge D" display unit. It plugs into the engine/trans diagnostic plug under the dash and gives you the readings that the ECM sees. Then you will see the "actual" trans temp, engine temps, ect., and it reads fault codes.