This problem has been throughly dissected over on Expedition Portal by Doug Hackney and others. Go to the Forum /General Expedition Camper Discussion and Modification/pivoting frames and mounting campers sticky at the top for more info than you could ever digest or need.
Since I still drive our XTC off road in some pretty traction free and off camber situations, this problem has always been on my mind. Our Lance has the factory equipped spring loaded tie downs in front and bumper bending non-spring loaded tie downs in the rear. I have the tension dialed in, but the tightness/looseness changes with the terrain and I make a decision about tension as it comes to allow the rear of the Lance to 'float' on one end or another if the frame and bed twist too much. I'm just trying to keep the Lance from twisting along with them.
Here is what I've learned about this. All pickup truck beds are hard bolted to the frame of the truck, usually with 4, 3/8" bolts using the shear of the sides to keep it from flexing too much. My pass side front bolt pulled right through the hanger so I used a bigger, thicker fender washer to keep that from happening again. But this pointed up the problem of stress/twisting and what kind of havoc it can produce. It depends on how much your truck frame actually 'twists' when on undulating terrain as to how much the truck bed will be subjected to twisting. The longer the frame of the truck the more it is subjected to twisting. Ford had historically the most flexy frames in the biz. I think they've seen the error of their ways and stiffened them up. Our short bed Q cab Dodge twists very little, so I only loosen the rear tie downs if it looks like the frame is going to twist, like hitting a steep sided ditch at an angle. I always try make my approach to keep the axles parallel and not twisted up even if it takes some aligning. Again, the last few years, folks have not had much of a flex problem with short bed, short frame trucks.
The 2,3,4, and 5 series trucks made the last 4 or 5 years have a LOT stiffer frames than previously. Do a search on the subject and myriad articles will appear.
If the Douglas work body is stiff enough it in itself will defray the frame twisting a bit. It's hard to know whether Douglas has done their homework on this subject or it is an, "out of sight out of mind situation." But it's good to have some education on the subject, and if you do go the Douglas route, be sure to quiz them on the subject. Douglas has a fine reputation for after market truck bodies and theirs will have a lot better shear factor than a stock truck bed. Our electrician has a Douglas truck body built specifically for a truck camper on his 2001 Ford F-350 SRW truck. It is a work of art, IMHO. Be sure the tailgate opening will accommodate a camper.
If you don't plan on any hard core off-road running in your new 4 or 5 series truck, I don't foresee a real problem here.
Another idea is to have an aluminum 'pivoting frame camper flat bed' with storage boxes made for your proposed new truck.
jefe
'01.5 Dodge 2500 4x4, CTD, Qcab, SB, NV5600, 241HD, 4.10's, Dana 70/TruTrac; Dana 80/ TruTrac, Spintec hub conversion, H.D. susp, 315/75R16's on 7.5" and 10" wide steel wheels, Vulcan big line, Warn M15K winch '98 Lance Lite 165s, 8' 6" X-cab, 200w Solar