What you need is a RC controlled on / off switch.
These are used to control items such as lights on RC Cars.
I searched on line and found some suitable for up to 30 Amps.
Make sure that the unit has separate input and output terminals to connect a Battery and Load for your device.
My device pictured is rated for 4 amps.
I have not tried to connect this device with two batteries, one for receiver/ motor and another battery for aux device, but should be fine. if it does not operate, try connecting the the two battery (-) terminals together with a wire, this will provide a return path to each battery.
Hope this helps.