In triangle \( TRS \), the centroid \( Z \) divides each median into two segments, where the segment connecting a vertex to the centroid is twice the length of the segment connecting the centroid to the midpoint of the opposite side.
Given that \( VZ \) (the segment from the centroid \( Z \) to the midpoint \( V \) of side \( TS \)) is 6 inches, we can determine the length of \( RZ \) as follows:
- The centroid \( Z \) divides median \( RV \) in a ratio of \( 2:1 \).
- This means that \( RZ \) is twice the length of \( VZ \).
Since \( VZ = 6 \) inches, we can find \( RZ \):
\[ RZ = 2 \times VZ = 2 \times 6 \text{ inches} = 12 \text{ inches}. \]
Thus, the value of \( RZ \) is \( \boxed{12} \) inches.