We are glad to answer any questions that you might have!
RUT950 does not support the output of PoE, it can only be powered via passive PoE on the LAN1 port only.
Thus to power the cameras, you'd need a PoE switch. Since the cameras are rated for 48V, I assume they are using 802.3at/af (active PoE), as this is in the range of the voltage for these standards. So the switch would need to support active PoE.
From our lineup, I'd suggest considering our TSW101, which can be powered by 9-30VDC, and outputs the active PoE voltages. It supports 30W per port, and 60W maximum total, so it should not be an issue to power these cameras. It was designed with vehicle use in mind, so your use case is perfect for it. We have a similar one described here.
If TSW101 is the switch you will choose, please keep in mind, that it should not be plugged into the LAN1 port of any Teltonika device as described here.
And as for the data transfer, that is correct. PoE uses unused pairs in Ethernet cables for power, and the rest are for data, thus only one cable is needed.
Let me know if any additional information is needed!