Is "True RMS" applicable only to measuring AC voltage?
Usually only AC ranges, which are AC coupled so you lose the DC component of the signal. The reason why you would not want the true-RMS to apply to the DC voltage range is that the accuracy of true-RMS measurements is usually not nearly as good as averaging DC measurements. 

For example, the Fluke 170 series datasheet here, shows these specifications:



The True-RMS AC measurements are almost two orders of magnitude less accurate than the DC measurements. 

A bit higher end is the Agilent 34401A 



You can see that the accuracy is more than an order of magnitude worse for the True-RMS AC range. 

Practically defining the high end, the Agilent 3458 offers several AC methods of measuring RMS AC. Even the best (and most restricted) True RMS AC measurement technique is more than two orders of magnitude less accuracy than the DC ranges (a few ppm). 



Here's a one-chip RMS converter, the LTC1966