Greetings Hlkoring:
We were in a similar position to yours 2 years ago. Went out and bought a new 2500 Dodge in 2010, before we nailed down the precise RV model we planned to buy; my thinking was that "this 2500 will handle anything we want to pull." Those were our "last infamous thoughts" before we started seeing RV models that appealed to us more. Another way of saying that is "we started seeing RV models that were longer and heavier." Long story short, in spring 2011, we decided that our new RV was to be Heartlands Big Horn 3455RL. This RV is very close to the BH3685 you are considering, in it's weights.
Anyway, back to the Dodge dealer we went; bought a new 2011 3500 Dodge SRW last spring, and then bought a 2012 BH3455RL last fall. We tow this RV with ease; as in, we hardly know its back there. Everything rides smoother with the 5th wheel attached, than w/o it; absolutely no exageration here. We completed a 6900 mile trip in November last fall, uneventful except for the sights and scenery, and couldn't have been more pleased with the handling and stability with the only addition to suspension on the truck being a set of Firestone air bags on the rear truck axle. If I do this again at some time in the future, I would have disc brakes put on the trailer axles. The drum brakes on our RV is the weakest link in this towing setup.
Our mileage in the first 6000 miles on the truck while pulling the RV was just under 10mpg. We currently have 16K+ on the truck and most recent pulls down to Oregon on 2 ocasions resulted in 10 to 10.2 mpg. Hitch is a non-sliding Pull-rite super 18K. We considered the dullie on our second truck purchase, but decided against it, as driving on some city streets might get a little uncomfortable with the DRW's. When we pulled up through mountains in southern CA heading back to Seattle last fall, we were in side winds in the 40mph range, and had nary a problem. Might have been a little poor judgement on my part, going past the overhead highway reader-board indicating "caution, high winds, tall vehicles not recommended in this area", but I did it anyway. Got lucky, and, that section of winding, windy road resulted in zero handling problems. DW and I are pleased with this towing setup.