What wood size for extending bed off trailer for "foamie"

[linuxmanxxx]

I built 6x9 on 4x8 trailer cut fenders off upsized to 13"wheels built storage below to clear the wheels using vertical 2x2 pieces in 1x2 stick frame to support cabin weight. Floor roof and walls were all 1x2 sandwich with 3/4"foam inside. With frp exterior and bit of over building was 1200 lbs queen bed tv built in ac galley with fridge. I could walk on the roof after completed.