You're doing the math right but you're looking at the GVWR and most TC owners are over that. We typically go by axle and tire weights. However, I see no real disadvantage to getting a 3500 for your application. Why not get one?
My own 11' camper fully loaded for a trip with my family in my crew cab dually put's me 1,600 pounds over my GVWR but I'm not over on axle or tire weights. I don't have a basement model and I don't have slides so even though it's a long camper it's not as heavy as many others on the road. I would personally never want less than a 3500 for a truck camper. I used to haul this same camper on a 3500 SRW, I was 1,200 pounds over the GVWR with that one and it was a regular cab and I had no children yet.
