How to get single new line in a Rich Text Box to appear as single-spaced

Try this

rtx_report.AppendText(lclFileInfo.pathOnly + "\r");

In Xaml, set RichTextBox Paragraph property margin to zero. By setting to 0 will not add additional line spacing.

<RichTextBox AcceptsReturn="True">
    <RichTextBox.Resources>
        <Style TargetType="{x:Type Paragraph}">
            <Setter Property="Margin" Value="0" />
        </Style>
    </RichTextBox.Resources>
</RichTextBox>