2009 R1200GS ADV clutch failure
Bin Ridin made a good comment. Not long after I got my '09 GSA, there was a service bulletin about re-routing the clutch slave line. I believe they basically replaced a zip tie installed at the factory. I had the mod done and rode the then-new machine to California (from NC). On the return trip, my clutch lever had a mushy feel and finally gave up near El Paso. Short story even shorter, someone in Germany had put the wrong fluid in the clutch resevoir. I had to have the entire system replaced.
Since then, everything has been fine but I do have it flushed out periodically. If I were preparing for a nice long trip like you are (you dog!), I'd sure get it checked. It may just need to be flushed.