I, like Silversurfer, have been mounting throttle and choke servos as close as one inch to the ignition box, battery, and engine for quite a while. I don't know when or where the 12" rule came into play but I and many others can prove wrong.
99% of RF is caused by a bad plug connection, bad plug wire, or using the wrong plug. Then there is metal to metal harmonics. I even had a plane that had a metal hub on a metal axle on the tail and at certain speeds would cause RF. Then there is the hard to track down bad servo extension wire or connection.
What I am saying is don't blame the location of the servo, look elsewhere, If it is only happening at full speed, I would be looking at metal to metal connections, wire connectors, Look at your control rods and clevises, are they metal ?