S&s put tags on their camper with its weight that was accurate. Get a photo of the tag from the owner or if you can go see it.
Is it an 11DBS designation? I have brochures from S&S from around 2004 2005 in storage at my house but not too handy. Meant to try to look briefly today when I went up there for someone else but completely forgot. Don't count on it but I'll be back up there soon and may have some luck and if I do I'll let you know.
Yep, you will need a dually guaranteed (at least to be anywhere near the specs of the truck!). Our 2004 8.5SC (no slide) was about 2850 wet and 3500-3600 or so loaded for camping.