Accurate RMS voltage measurement of DC with RF ripple
OK, so from what you say 


You have RF on top of DC
You want to measure the DC voltage, averaged over "some delay", and ignore the RF
The RF makes your multimeter (or other equipment) misbehave.


If this is what you need, the answer will be a simple filter, for example a second-order LC filter with optimum damping, so L - R - Cap to GND. This will remove the HF, smooth the voltage, and average it a bit, depending on cutoff frequency. You could also use a notch filter, but this would not remove the harmonics very well.

Now, a filter will of course slow the transient response, so if the "DC" voltage you are trying to measure is not really DC, but varies at say 100kHz, then you'll want your cutoff frequency to be:


High enough above 100kHz for good transient response
Low enough below the 13.56MHz you want to remove, so it is actually filtered out.


I'll update when you give more info about this.