Sportsmobile trailer

I have started building a big weekender, 12 ft long 6 feet wide 5 ft tall with a pop top built on an old pop up trailer with working surge brakes. I will be hauling it with a full size pickup so while i am trying to keep it compact the weight souldn't be an issue. I realize this is probably on the edge of a tiny trailer and a full size behemouth but I know you people are the experts on building trailers. I have the sides cut out but not mounted, when i came up with another idea. I am thinking about mounting the bed on top of the trailer in a loft in the pop up area like the do with the expodition vans. I am wondering how much room I would need for the bed area, like is 3 feet enough space between the bed and the roof? Also how much room would a person need for a sitting area under the bed? I could cheat the cieling down a foot if 4 ft is high enough for the setting area and only have two feet of pop up room. What amount of support do you all think I would need for the loft section? 2X2 18 in on center with 3/4 be strong enough or over built? I haven't seen anyone take this on yet and I am sure I have just missed it.