Honestly, the all caps is not needed, especially when a simple google search of "measuring dcc rms voltage " gives you tons of answers.
so besides searching and seeing all the ways to measure it, connect a full wave bridge rectifier to the track, then a DC voltmeter to the output of the full wave bridge rectifier.
You could read this page:
DCC FAQs
OK, now to the why?
For us, we should know the DCC voltage to be sure we don't have wiring issues, voltage drop, etc. that affects operation.
Another reason is that certain decoders do not work at the higher voltages usually used on G scale (the NMRA says the decoder should be able to withstand 27 volts, I run my layout at 24 on the rails).
Now, finally why did THEY ask you? To be sure you did not exceed the rated voltage of the decoder.
I notice you changed from the Large Scale "Budda" rated at 5 amps, to the smaller "Black" decoder rated at 1.1 amps PEAK, not continuous, and I cannot find any specifications on their site as to max voltage... in fact no specifications. Clearly these decoders are intended for HO and smaller.
So what locos are you running from these?
Greg