For your 2023 Keystone Montana 3901RK with a gray fender skirt measuring about 76 1/2 inches, the recommended replacement is the OEM-style ICON Keystone Montana tandem fender skirt. Although your measurement is slightly longer, the standard 76-inch version is the correct fit and matches the OEM gray finish and mounting pattern. You can order this fender skirt directly from Elkhart RV Parts. If you need assistance confirming the correct side (left or right) or verifying the hole pattern before ordering, support is available to help ensure proper fitment.