If I was carrying a camper full time close to the SRW 18" aluminum wheel ratings of 3590x2 per wheel, I would probably get the 19" wheels.
It seems odd to me that, according to the Ford specs in that clicky, both the F250 and the F350 SRW would have a 7000 lbs spring pack with the truck camper package.
Being at the wheel limit of 7180 lbs on the rear axle on an SRW on a long term basis, even if the tires could handle more than that (like the Toyos or the Nitto 285 tires), the major limit there seems to be the wheels.
My rear axle weight with a non-slide camper fully loaded is 6500 lbs. I'm pretty comfortable with it. An additional 1000 lbs go on the front axle.