Both can power your SmartHub Gateway. However, connecting the Gateway to your RV's 12V power system provides the additional benefit of built-in house battery voltage monitoring.
You can connect to your RV's 12V system using either the included 12V vehicle power adapter or 12V direct-wire pigtail.
If you use the AC wall adapter, the Gateway will still operate, but built-in house battery voltage monitoring through the Gateway's power connection will not be available.