The servo outputs are no good because they are each only a single channel. You need a combined signal with all channels on it and you can't recombine the servo channels because of timeshifts in the decoders.
You need to find the combined signal inside the transmitter, or receiver if that's easier. In the Tx it will be somewhere on the input to the RF stage. If there's an NE5044 in there that's the encoder chip so it'll be easy to spot. In the Rx it will be the input to the decoder chip (possibly a 4017/4015) but it's not really easy to find in either case without an oscilloscope.