How can I animate a WPF Grid to slide in from the left and out to the left when Visibility changes?

Question

I have a WPF Grid as follows:

<Grid x:Name="Main Panel">

   <Grid x:Name="Detail Panel 1"></Grid>

   <Grid x:Name="Detail Panel 2" Visibility="Collapsed"></Grid>

</Grid>

And a style that expands it when visibility changes. But really I want it to slide in/out rather than expand and compress

<!-- Defined animation for appearance of Panel to slide in/out from left side-->
        <Style TargetType="Grid" x:Key="expand">
            <Setter Property="RenderTransform">
                <Setter.Value>
                    <ScaleTransform ScaleX="1"/>
                </Setter.Value>
            </Setter>
            <Style.Triggers>
                <Trigger Property="Visibility" Value="Visible">
                    <Trigger.EnterActions>
                        <BeginStoryboard>
                            <Storyboard>
                                <DoubleAnimation Storyboard.TargetProperty="RenderTransform.ScaleX"
                                                     From="0"
                                                     Duration="0:00:01"/>
                            </Storyboard>
                        </BeginStoryboard>
                    </Trigger.EnterActions>
                </Trigger>
            </Style.Triggers>
        </Style>

EDIT

I replaced the scale transform with a translate transform as follows, which sort of works except it seems the right side of the incoming grid is always pinned to the right side.

So how would I make the grid slide in with a fixed width equal to the parents width such that it appears like a page sliding in over the lower grid layers?

<!-- Defined animation for appearance of Panel to slide in/out from left side-->
        <Style TargetType="Grid" x:Key="slidein">
            <Setter Property="RenderTransform">
                <Setter.Value>
                    <TranslateTransform X="0"/>
                </Setter.Value>
            </Setter>
            <Style.Triggers>
                <Trigger Property="Visibility" Value="Visible">
                    <Trigger.EnterActions>
                        <BeginStoryboard>
                            <Storyboard>
                                <DoubleAnimation Storyboard.TargetProperty="RenderTransform.X"
                                                     From="-1000"
                                                     Duration="0:00:01"/>
                            </Storyboard>
                        </BeginStoryboard>
                    </Trigger.EnterActions>
                </Trigger>
            </Style.Triggers>
        </Style>