LF Low-pass filter for RF
You can ignore HF coupling all over the circuit, if you know that one particular spot in cicruit is immune. Build a fully closed calorimetric metal box/chamber (2-3 cm side) with single feedthrough cap, and load it with ordinary filter and thermistor. After this calibrate the thermistor using known sources and second identical thermistor for ambient. 

You will have the lowest parts count true RMS calibrated sensor. If you need higher precision (few tens/hundreds ppm) then you can employ the air-flow, oil-immersed bath, bath in vacuum, thermocouples, platinum black bodies etc.