If you just want to have a true RMS voltmeter, maybe you should consider HP 3400A. It can be obtained on ebay, with some patience, very cheap. Probably cheaper than something comparable you can build.
If you want to experiment, you coud try to use square law diode detector.
Square law diode detector is used in, for example, Ballantine 323 analog true rms voltmeter. This meter from early 1970s still has quite respectable specs (10Hz - 20 MHz, the lowest range is 100uV full scale). Schematics is available online. After squaring and rectifiying, the signal (I believe less than 500uV max) is chopped and amplified. It has electromechanical chopper operating at 94 Hz.
Ballantine still makes this meter. I guess it does not have electromechanical chopper anymore, but I couldn't find the schematics for the current model (they sell manuals for $100!!!)
I got one recently very cheap but the mechanical chopper is not working. When I got time I'll try to replace it with an electronic one.