For your 2018 Montana High Country 5th wheel roller shades measuring 64" and 73" wide, you'll need replacement slow-rise springs sized to match your roller tube diameter. Most premium RV roller shades use a 1.25" diameter aluminum tube, while some Valu-Line shades use a smaller 0.75" tube. First, measure the outside diameter of your roller tubes to confirm. For your shade widths, select the Large size spring compatible with your tube diameter: the 1.25" tube Large spring (22.5" total length) or the 0.75" tube Large spring (18.5" total length). These replacement springs come with installation instructions and video guides to assist you. You can order the appropriate Irvine replacement springs directly online to restore proper slow-rise function to your RV roller shades.