The replacement spring for the roller shade on your 2019 Thor Miramar 37.1 RV requires a roller tube with an outside diameter of 1.25 inches (1 1/4 inch). This is the most common size for premium RV roller shade springs. You can order the Irvine Replacement Spring (1.25" Version) designed specifically for these shades. If your roller tube diameter differs, other sizes are also available. To ensure compatibility, measure the diameter of the metal tube your shade rolls around and select the spring accordingly.