For your 2015 Airstream Interstate EXT, the OEM replacement fresh water tank with part number 602184 and dimensions 12" x 12" x 48" (26 gallons) is the best option to ensure proper fit and function. The molded indentations on the tank are critical as they allow hanger bolts to pass and securely attach to the support bracket. Additionally, the tank includes a heater pad wired to the bottom, which should be considered when sourcing a replacement. To confirm compatibility and identify the manufacturer, it is important to provide photos of any stamping or markings on the existing tank. If access is challenging, scheduling a mechanic to lift the RV and capture these details is recommended. Once the tank manufacturer and specifications are confirmed, Elkhart RV Parts can assist in sourcing the exact OEM replacement tank that matches your vehicle's requirements.