The B300 & B300S batteries have onboard 12V DC output, however you cannot monitor the power usage of those ports even when they are connected to the AC300 / AC500 units.
I use a small 300W AC inverter connected to the 12V port on one of the B300S batteries because that’s much more efficient than running the 5000W inverter. The problem is the Bluetti App and the AC500 unit themselves don’t display any usage of the port when it’s in use.