I have two twin screw boats that are both built with one battery/ECS for each motor, and in each boat one battery and ECS provides the power for the receiver and servo. One boat is a tug that has two 12V 7Ahr sealed lead acid batteries with an ECS each, the other is a warship that uses two 7.2V NiMh packs and with both of them I've never noticed any difference in the charge levels of the batteries even after serveral hours of running. The current load drawn by the receiver and servos is tiny, under 100mA, this is a negliable current load when compared to what the drive motors draw, I wouldn't worry about it.
The other ECS in each boat has its middle, 5V supply cable disconnected from the receiver because its not needed, but you can still put that 5V to good use if you know how. The voltage regulator in the ECS that supplies the 5V is capable of suppling up to a 1A load (Mtronics ECS are certainly capable of this), this is plenty for powering LED lights for example. In the tug I have this ECS connected to the receiver through a circuit board that I built that allows the black ground and signal cables to pass through to the receiver but terminates the 5V at some connectors that can be used to connect LEDS too. The circuit board has the correct resistors on it to protect LEDs and a relay switch circuit so that the LEDs can be switched on/off from the transmitter. The total current load of the LEDs and switch circuit is under 50mA (thats 0.05A), negligible to the battery.
Hope this helps.