Sorry if this is long but hope to present a complete picture as best I can.
Vehicle: 2006 Chevy Express 3500 RV Conversion (Roadtrek 210P). Weighs 9600lb, right at GVWR
153k miles. Fluid/filter changed at 60k and 110k
Valvoline Maxlife Fluid, Lubegard Transmision Fluid Additive, Magnefine...