Jack
You are correct the dash gauge is a stand alone gauge wired to a signal wire temperature sending unit which is usually screwed into a tee fitting on one of the transmission cooler lines this is a grounding sending unit so the tee fitting must be grounded for the gauge to read properly.
Where as the Silver Leaf 200 unit is reading serial stream data provided from the transmission TCM this temperature reading is taken from a temperature sensor located as part of the transmissions internal wiring harness, you should be able to display any transmission fault codes using the shift console there is a full troubleshooting manual in the coach assist section of the forum which will help exp!ain the troubleshooting procedures to help you confirm a d repair your problem.
Start by using the shift console to retrieve any Transmission fault codes and go from there.
Hope this helps
Eric