Nice looking progress.
I think Steve is pretty much spot-on regarding twist. The heavier the load and the more likely twist will happen. I got to arguing about this alittle bit with a guy on another forum and he took me up on my suggested challenge and drove one wheel up on a block or something and he showed pics that looked pretty clear it wasn't twisting at all. I kind of gave up on the argument but he had a tiny camper on an F550 and over time I have become convinced that was the reason. The frame on my Kodiak is way stiffer than the F550, yet mine twists a lot over uneven ground. The difference is I am carrying about 19,000 GVW of which maybe 8K is the bed and camper.
I should grab a picture of my Kodiak shop manual (or is it the body builder's guide?) because it actually has the recommended design for super inflexible loads such as a water tank. The more inflexible the load, the more you might need a flexible mount. The design actually strikes me as simpler than many complicated setups I have seen such as on the expo forum, but basically a strong spring between tabs on the frame and bed in the front and solid mounts at the back. You might check the body building guide for your chassis if there is one.
The original builder of my rig actually set the camper in the bed on some sheets of plywood (the cab is extra tall so it had to be lifted) and then used a forklift to lift one front wheel....observed the amount of flex and then added or subtracted enough sheets to have the proper clearance. Then he mounted it using normal spring loaded turnbuckles from Happijac. I switched them out with Fastguns quickly. But he thought the steel flatbed already flexed some, and there was no need to do a special bed mount. It was flexible enough to help drive on rough roads, but not so flexible he worried about the camper holding up. In the end the simple solution worked for him on a 1-year trip to South America and back, and us for another 5 years of camping for a total of about 2 straight years of camping full-time and the Lance has held up nicely. Your structure will more rigid than the Lance which could be a good or bad thing depending on how everything works together. I think it is probably wise to plan on not doing anything special to begin with, but closely monitor the tie downs to begin with and make a change if necessary later.
Great looking project - I am jealous; I'd love to do that someday.
