In almost all receivers, the power lines (+/-) are just a connected bus that is powered by the BEC on the ESC. You're not placing any load on the receiver, just the ESC or whatever is powering the receiver.

Check the specs of the BEC in your ESC and map out the load from other devices like the servo. Another option would be a separate buck converter running direct off your main battery. With appropriate wiring/electronics, you could still control it from your receiver.