Like you said, if it knows it's off then it's not a GPS issue. So I think the RTK, CORS, etc are fine. 

I'm not familiar with how red tractors steer and sense the steering especially a QT but it seems like the valve and/or sensors would be the first place to look. 

Whatever the equivalent to a wheel angle sensor is on a track machine. The valve itself, any wiring to/from those and the items themselves. Guessing machine hasn't thrown a steering code?