it would be much easier to buy a 40' flatbed goose neck trailer and then take the axles off your tt and place it on top of the flatbed and weld it to the frame along with some orher kind of hold down system. there are like 5 of them around here where i live they use them in the oil field for portable offices and sleeping quarters.
Just saw one two weeks ago at Cedar Key, Florida, on a construction site. It was a Airstream or Avon style and totally scrapped clean below the skin of all axles, but the frame had it secured. I can say it looked unigue. ;-=
Sweet.If only it had a pass through window in front to save walking all the way back inside to get a drink. Like the little touch of home picket fence.
LOL, the picket fence is actually an aluminum stake side that doubles as a loading ramp. The left side one is still under construction but will be in place later.
I like the idea of a passthrough window but the bed is in the way.
And before anybody asks, this is a full frame gooseneck trailer, not an addition added onto the travel trailer frame. I left the TT frame alone other than cutting the stuff off the bottom to weld it onto the new frame. The axles are 7k torsion's with 16" wheels. The corner jacks are reused and the plumbing drain was cut then modified to route under the new frame. In the process I added more belly insulation when I reinstalled the liner. These are the only modification to the original travel trailer that I made.
I took this thing across the scales and the trialer axles have 8660, and the tow vehicle had 2940 for pin weight.(empty weight of 11720) This is the big reason I went and bought the dulley truck. When the 75 gallon diesel tank and the 40 gallon gas tank get filled (up on the gooseneck) and then the motorcycles get loaded onto the front deck I was going to be waaaaay over 3500 lb pin load. Not pictured are the 4 6volt batteries but they were on the trailer when I weighed it.