by Arne » Thu Nov 13, 2008 10:13 am
the hatch is only a weak point when open. when closed, it is fully supported by the wall.... if you do it correctly, I see no problem with it. I built mine very similar to what you are doing... I really like the straight cuts on the hatch joint. much easier. see 4th pic on page 7, link at bottom..... if you are still concerned, put a doubler at the angle in the hatch. That, I would do, some 1/2" ply for extra strength.
The wall is also bolted to the chassis, so 4" seems excessive to me. 3" C channel is what I would go with.. but that is me..
Remember you have a wall/ frame that becomes one member... and if the tongue weight is only 100# or so, I see no reason for 4" tubing.