You won't be able to stay under the GVWR of that truck, with either the Grand Design or Cougars that you mentioned. I didn't look at all the others specifications, but assume they are in the ball park.
You weighed the truck, but did you get front and rear weights? I would suggest you will be okay if you stay under each axle weight rating. There is no reason you should not be able to haul a 10k GVWR fiver with your Ram, which is the gross weight of the Grand Design 29RS. I think about 10k is where you want to be on a 3/4 ton truck.
In the Cougar line, suggest you take a look at the "Half Ton" or "Xlite" series. Those match up well to a 3/4 ton truck. Not really 1/2 ton suitable, in my opinion. I'm pulling one of those with my 3/4 ton. Good value priced trailer, and the 3/4t truck handles it with ease.