Truck towing question

I do expect awful mileage, as per old trucks. I just don't know trucks or towing very we'll. Wasn't sure if I'd be pushing breaking something pulling a camper. Heaviest thing if pull might be 5-6k lbs, but mostly if pull my Dart or down the road the old Charger I'm (hopefully) getting from my uncle, down the road. As far as campers, I dnt expect ever owning anything larger than a tent-trailer or 1 of these hybrid tent/travel trailers, which aren't terribly heavy. The only thing I know about trucks is my dads 1/2 Chevy Acalanche is "rated" roughly 8k lbs with a 5.2 auto. That, and I know Cummins tows over 10k without breakin a sweat