The old doors were made from 3/8" and warped on me. We stayed dry during a 12 hour rain, but it did get a little wet while driving 7 hours in the rain.
The only thing I hate about building something, is having to build it twice.
This time I did it overkill.
Primered and ready for paint.
I used 3/4" lumber for the sides, and 1/2" birch ply for the inset. The inset actually sits inside the old door jambs (I made an aluminum frame for it) and the outer door sits on the outside of the door entrance. I used the router to round off the outside of the door, and I used it to cut away the material for the inside weatherstripping. It has weatherstripping on the door jambs, as well as the outer part. The doors are sealed twice with a drip rail above that.
We're not sure if we'll add a window to this door or not - the other door hasn't been changed out yet. We're considering making a faux door with a window on the opposite side.
Well see how it does - We're going camping at Tyler State Park tomorrow, and then I'm off to Caddo State Park on Monday & Tuesday.
Steve
I need to add some rivets. Actually, it looks better black, and it will look normal with a window. As normal as a hostein teardrop can. 
