Hi Zach, Besides the mentioned Handbook of Texas Online (Texas State Historical Association) reference that was quoted on Rootsweb, I will also refer you to the Daughters of the Republic of Texas biography book that you can see in a Google Books search:
It notes that his family later lived in San Patricio and Edwards counties.
As to what he was doing around Austin, what exactly do you mean? Why did he die in Austin/Travis when he lived in Milam Co., or why did he stay in Texas?
You might enjoy seeing documents relating to Calvin. Check the Texas State Archives, http://www.tsl.state.tx.us/arc/genfirst.html#republichttp://www.tsl.state.tx.us/arc/genfirst.html#republic and use the database to find numerous Republic claims where either he is mentioned or he posts a claim.
The Texas General Land office also has a database of Land Bounties. He's listed as getting land in Milam and Bell counties.
One final note: it's possible his Bell Co. land was around Salado. The Robertson family settled in Salado very early (The Robertson ranch is still in Salado). If so, there was a major stagecoach route running north/south through Salado. Maybe that explains the death in Travis? He was traveling to the state capital?