I'm likely to shed more confusion than clarity on this issue with this note, but let me throw it out there anyway. When working with Microsoft Word, you can specify fonts in a very direct fashion (highlight some text and specify a font and a point size) or you can specify fonts and point sizes in a more indirect fashion by using styles. I understand quite well how styles work in some publishing environments (e.g., HTML and TeX), but I have to confess that my understanding of styles and Microsoft Word is lacking. I have had trouble figuring out how styles work in Microsoft Word, and I have had trouble finding good tutorials and help files about how styles work in Microsoft Word.
I bring all this up because RTF files produced by RM seem to be more oriented towards specifying fonts and point sizes indirectly via styles than directly via fonts and point sizes. So you may need to look at how styles are being used in your RTF document, assuming you have more luck than me in figuring out how Microsoft Word (or WordPerfect X6) deals with styles. Ideally, Renee's suggestions will solve your problems and you won't even have to deal with styles. But if Renee's suggestions don't solve your problems then styles are something you might want to look at.
Here's one more thing on Microsoft Word and styles. I nearly always do some post processing of RTF files produced by RM to clean things up a bit. After having done so, I have to rebuild the indexes so that the pagination in the indexes is correct. My issue of needing to deal with Microsoft Word's styles sees to be more significant after rebuilding the indexes than it is before rebuilding the indexes. I wish I could be more complete and accurate with my comments about styles, but so far the score on styles is Microsoft Word 1, Jerry 0.