Accurate RF voltage magnitude measurement
"Not loading too much" is vague : any estimate of the effective source impedance would help.

I would consider a parallel resonant circuit tuned to the known frequency : its impedance can be very high so it will not load a (steady state) sinewave source. (You did say high purity, so I'm presuming it's unmodulated).

Now you need a lower amplitude output to measure using conventional techniques. This could be a capacitive tap as user44635 suggested (the L simply resonates to tune out the capacitance) or a tap on the inductor, which will act as an autotransformer, transforming the impedance of your measurement system load by n^2.

edit : an inductor's value can drift with temperature (as can a capacitor's), but its turns ratio won't drift. This will affect the tuning frequency and that will have a quantifiable effect on the impedance (hence loading) at the frequency of interest, so this drift is a second order effect on accuracy. You'd need to calculate the loading effect as part of your error budget. Alternatively you could re-tune to find the peak, either periodically or for each measurement.

Clearly the turns ratio cannot drift : how accurately that translates to voltage ratio also depends on the coupling. I don't know how to put numbers on that, but if the secondary in a tap on the primary (autotransformer) I think it's pretty good.

ALTERNATIVE VIEW : do you need absolute accuracy or relative accuracy? It strikes me that most traditional techniques for precise measurement use differential techniques : between two arms of a Wheatstone bridge, or subtraction of a precision voltage (Daniell or Weston cell etc). If you had two nominally identical voltages (frequency and phase locked) can you measure the difference between them?