Robin only uses the voltage for the power calculation and to determine the zero crossing point, he does not report it. It would be possible to extract the voltage, but it’s probably easier to use emonLibCM and rfmTxLib - these are proven so there is likely to be less debugging required.
I think your starting point will be the example sketches that come with emonLibCM - particularly the one with the radio: EmonTxV34CM_min_RFM69.ino – but this is RFM69CW only, so you need to remove the rfm.ino file and use rfmTxLib instead. You will need to make some minor changes as a result.
[Note, the “max” sketch is only there to illustrate every setting for emonLibCM - but it changes nothing because those settings above and beyond the “min” sketch only set the default values again anyway.