@ksjp:
It would look exactly the same as the front-facing one you have at the top of the screenshot. Simple square roof.
The 2.5 perspective is merging the front with the birds-eye. From a birds-eye view, the slope doesn't matter. You would see the rectangle just as you do in the front-facing outhouse.
---Edit:---
Just in case this still seems confusing...
It comes down to geometry and perspective. The whole RTP 2.5D perspective can throw you for a loop for a bit, but it helps to remember that it's two separate perspectives joined together.
A good way to explain it is with a 3D shape like a cylinder. When you look at a cylinder from the front view, it looks like a rectangle. But if you look at it from bird's eye view, it looks like a circle. So when drawing this in the RTP perspective, it would be a circle at the top and two straight sides (front rectangle).
Now, it might seem a little more confusing when you have a slope, but it's actually still really simple. Whether the slope is facing toward you or away from you doesn't matter in the bird's eye view. it will still look like a rectangle. You will be able to show the slope in the side view, but the front and back will look identical.
