The fit issue with the 30-59" replacement spring for your 2017 Thor Challenger RV is likely due to a size discrepancy in the keyed plastic pieces that interface with the 1.25" aluminum tube. Although springs manufactured from 2017 onward typically fit snugly without modification, some 30-59" springs may have thicker keyed plastic teeth that do not align properly with the tube's slots. This can prevent the spring from seating correctly inside the tube. To resolve this, lightly sanding down the plastic teeth on the spring's end gear can help achieve a proper fit. This adjustment is a common solution for ensuring compatibility when springs do not fit the keyed slots as expected. If the spring still does not fit after modification or if you prefer not to alter the part, contacting Elkhart RV Parts for a potential return or exchange is recommended.