Is your DMM expecting a sinusoidal input? The RF300 output may be far from sinusoidal and confuse the meter if so. Can you confirm the frequency using a 'scope?
Is the RF300 frequency affected by the supply voltage? What is the peak-to-peak or average/RMS voltage of that frequency signal?
I see from the RF300 spec that its only connections are via a screened twisted pair cable, so presumably it relies on a phantom power supply. Not sure how that will affect the simulation using Mike's circuit.