First off, the R7008SB RX sends receiver bus voltage without using the added sensor.
Second, if you want a second battery voltage (like motor, before regulator, etc) then all you need is the cable CA-RVIN-700.

The SBS-01V sensor allows you to monitor a third pack.

SO:

RX = servo bus voltage. If unregulated = RX battery voltage. If regulated = regulated voltage to the RX/servos.
EXT Battery = either voltage before a regulator, if none then you can get motor pack voltage
SBS-01V = Third pack. I would only use this if I was using a voltage regulator for the RX power and wanted to get the following:

1 - Regulated voltage to RX
2 - Unregulated RX pack voltage (before regulator)
3 - Motor pack voltage

If you are using unregulated RX power then you do not need to use the SBS-01V.

In your case I would need to know how you have things connected to diagnose your issue.