Are there any reasons, other than the weight of the camper, that you are looking to change? High Mileage? Year of the Vehicle? Just tired of it and want something new?
Cause basically, I can't help wondering what it would cost to yank the SRW set up out of your truck, and drop the correct DRW axle and springs in? You would need new fenders, which could be sourced from a junkyard. As could the axle itself and springs, if you could find a good one. Might even be able to use an axle and such from a 4500 series? (Does GM make one? I know Ford and Ram do, not sure about GMC.)
The other alternative that occurs to me would be to take your truck and fully loaded camper to someone like Deaver Springs and let them engineer new springs that are correct for the weight. I kinda think that would be better than a hodge-podge of aftermarket parts that were never designed to work together.
Just thinking out loud and playing Devil's Advocate here . . .