A separate RX battery should not be needed but you still can opt to use one if desired.
It is much easier to estimate maximum RX operating time with the separate RX battery. The only indication you will have of time remaining in the power system battery is the low voltage cutoff if your radio system does not include telemetry downlink of the battery voltage.
When using just the power system battery with BEC, the radio will function long after the low voltage cutoff prevents the motor from operating. But continuing to fly for very long after the cutoff will lead to damaging the battery. Damaging the battery is more of a concern than losing control of the model.
This is a place where your flying style will be very important in the choice.
If you opt for a separate RX pack you could go with a LiFe pack which would save weight.... or if you need the nose weight anyway the older NiCd/NiMh is still fine.
We used to fly models similar to the Gentle Lady with brushed power systems and NiCd packs made of 6 sub-C cells (old RC Car packs) and the separate NiCd RX battery. The sailplane models this size are capable of passable performance even with that much weight on board. The brushless power systems and modern mini/micro servos allow the electric powered versions to now be lighter than the pure sailplane versions used to be... So don't be too concerned about the weight of the separate RX battery.