When I spoke with a dealer about the 865 vs the 990 I was told it really doesn't make that much differenec. Weight wise you will not be a lot lighter (they list ~365lbs lighter than the 990) though perhaps they have the smaller tanks which would keep you from loading it up as much I suppose.
If you want to stay under GVWR than you need to look at a SB Dually. Otherwise, SRW trucks aren't rated high enough to carry these campers and stay under the limit.
Personally, I've ignored that, added air bags (a lot of duallys do to), stable loads and 4080lbs tires and said "to heck with it"