Surge Guard 40250-RVC and 40350-RVC are interchangeable. Buzzing usually means loose connections - tighten all terminals. If buzzing persists with tight connections, the relay may be failing and unit needs replacement.
Built-in Southwire surge guard EMS provides comprehensive protection including surge, voltage monitoring, and fault detection. External unit unnecessary if built-in is full EMS. Verify yours has EMS features not just surge protection.
Weatherproof covers for surge protectors available from TRC, Camco, and general electrical supply. Use in-use style covers that allow cord exit while protecting from rain and sun.
Auto-reset EMS in 180-250 dollar range is worth it for 30 amp RV. Protects against surges, low and high voltage, and wiring faults. Progressive Industries EMS-PT30X and Surge Guard 44280 are top recommendations.