Just a thought, but I have a trash hauler, with a single trailing arm(each side), coil, independant suspension. I took the whole rear sub frame, off of, a front wheel drive Van. Any load from empty, to overloaded, it pulls better than any leaf spring type trailer, I´ve ever tried. I´ve layed tools in the back and had them still be there, when I got home (driving empty) and have run loads around a ton. Works really well on the interstate. Bonus is it has a 4 1/2 inch bolt pattern.
The frame is a big T, with the cross of the T being the mount for the sub frame and suspension. Frame could be a + , for extra strength. A few ribs and your finished.
My bed is a sheet of sealed (polyester coated) 3/4 inch plywood. Might rot somewhere sometime, mine is over ten years old. Don´t under estimate plywood, a few bolts (large washers) and a sturdy frame, make a really durable platform.
As a side note, I´ve used plywood (12 ply) for skid plates, actually works better than steel or aluminum. Won´t bend, won´t dent, has a really good memory (springs back to where it was), easy to replace. Used it for a sub frame on VW Buggies.