The salesman sent you on your way because he made a sale. Or, to give him the benefit of the doubt he's just ignorant of the true camper weight/ truck ratings.
You're not missing anything; truck campers are HEAVY.
You can modify the truck you have to make it work but it will take a lot of time and money. I used to haul my camper on a 3500 SRW. I weighed in at 11,140 on a truck with a GVWR of 9,900. I had added air bags and HD shocks and drove it thousands of miles. I now carry the came camper on a dually. I weigh in at 13,100 on a truck with a GVWR of 11,400. I've added air bags, HD shocks, and the tires aren't stock. I've driven even more thousands of miles.
