I would stay under the 5k dry weight and get closer to 4k if you can. I pulled our Jayco (around 5k dry) the first season with a 2007 Silverado 5.3 (4 speed) and it I dreaded any moderate inclines as that combo just seemed under powered. I would have to manually downshift at certain times or the transmission would constantly hunt to find the right gear. The newer 6 speeds seem to have resolved that issue but I think (given your elevation and current TV) you would be unhappy towing a TT close to 5k lbs....unless you don't mind hearing the engine rev like it going to blow and going 30-40 mph up a good incline.