Andrew,
I am rehabbing an older travel trailer. Its frame consists of a 3" channel with angle welded to the bottom. There are (3) 2" angles that run across the trailer to the outside wall (84"). only one is behind the wheel and there is no channel connecting the 2 main frame rails. There is one additional angle inbetween the 2 ahead of the wheels, but it only connects the main channel. The main channel is bent to form the A frame.
This trailer is from 1970 and the frame is plenty strong. Of couse the main channel is likely much larger stock than a bolt together trailer.
The trailer cabin floor (orig 1/2" ply which I made 5/8") rests on top of the channel and on 2x4's cut to the height of the channel (about 3" tall) which are resting on the angle. The trailer cabin is secured to the frame with lag bolts (yes and only 9 - that's just the way they did it) bolted through the angle. It does not have a dropped floor but it easily could - and have considered doing so as it has a max interior height of 6' -I'm 6'4 so I'd have to stoop regardless so I am still contemplating if it is worth the time an effort for 3".
In the second drawing, I have tried to make a sectional view of this construction. the red is the frame rail, the blue is 2x4 trimming to height of the frame rail, and there would be angle secured to the bottom of the channel. The trailer cabin floor (non dropped portion)would be resting on the main channel or 2x4's, and the dropped section directly on the angles.
Like you said, it will probably be strong enough, but I'm not an engineer so that's why I posted it as an idea.
If it works, it might be a neat way to build a rear door Scotty.
Mark
............of standy room...
