There is a temporal solution - you need to compile you application with the define RICHVIEW_DPMARGINS (for example, place {$DEFINE RICHVIEW_DPMARGINS} in RV_Defs.inc - of course, a source code version of TRichView is required)
Margins will be read from RTF in RichView.DocProperties, in stings like
'LeftMarginMM=20'.
You can then assign these values to properties of RVPrint.
This solution is temporal because DocProperties was designed to store custom user properties, not standard document properties like margins.
Initially TRichView was not designed as WYSIWYG editor, it was assumed that page layout properties are not document but application properties (the same for all documents). What's why TRichViews do not have page layout properties, they are concentrated in TRVPrint component which can be used by many TRichViews.
It will be changed in future.