You have a 3500 then with an upper overload spring correct?
The upper stable load will engage the overload sooner but I don't think you'd cut the sag in half. Maybe reduce it by an inch or 2 only? I think the overload will pick up just as much weight and not a bunch more only being a single shorter leaf.
Quick way to test it. Cut 8 little blocks out of 2x4s and stick them in there and see what it does. Or whatever thickness the stable loads are.
Betting upper and lower stable loads or some Timbrens or airbags with low pressure will be needed to get it where you want it though.
Those campers are as heavy as they get fr the size of them. Same as the old 860 models I have/had and although I've never weighed the camper, based on air pressure in my bags if I air up to empty ride ht, I'm between 4-4500lbs ready to camp.