Sundowner trailers are very well built. The total height of 12'5" is not bad, most fifth wheels, buses and truck conversions are right around there too. Buiding a ladder style designed reinforcement should work, the reason why I asked is that most sundower trailers floors are made of an aluminum extrusion.
Looks like you have a lot off money into your trailer, I would really think about it and make sure its planned out before you convert it.
My trailer I designed and built it and have less than $7,000 into it.
Photos are at:
www.geocities.com/rosenbergwelding/
The problem that I have is the trailer weight, towing 16,000lbs up a mountian with a 1-ton truck does not work well. So I modeled the the truck and trailer, will at basement storage compartments and change wheel wells, change the trailer front and it will look pretty good. The total height will be about 11'4", will have basement ac and heat. I would like to use a cabover tractor, I know they do not ride as well, but they are not as long and I will have some extra storage space.
Looking for some information on chassis stretching, photos or anything else would help.
