An Android head unit is an excellent choice for running RealDash, mainly because they solve the power management, boot pretty fast and have very low standby power consumption.
There are multitude of vehicle specific Android head units which are able to show vehicle data, like speed, indicators etc. But it seems like RealDash is not able to read that data.
This is true. These Android devices do not offer any API for 3rd party apps to access the vehicle data the same way as internal apps do. Some of these devices (but not all) use SocketCAN connection internally, so in some cases you can use SocketCAN connection type to read vehicle CAN bus.
To make matters worse, most of these devices have partly disabled Bluetooth implementation. It seems like normal Android Bluetooth stack has been removed and replaced with manufacturers own that allows to use your phone in hands-free mode. As a side effect, you are not able to pair traditional Bluetooth devices with the head unit.
If your headunit does not show any data in RealDash CAN monitor with SocketCAN connection type, the best option is to use USB CAN adapter. See supported adapters here:
RealDash | Manuals | Supported CAN/LIN Adapters
If you do not have your vehicle CAN information available, an USB OBD2 adapter can be used instead. Data rate will not be even close as fast as with CAN, but it is easier way to read data from your vehicle.
As for OBD2 connection, we recommend adapters from OBDLink:
What Adapter shoud I use? - Connectivity / OBD2 - RealDash Forum