Light weight is desirable, but light pin weight is what you need most. Our Rockwood 8280ws has a pin weight of 1240 lbs loaded, which is 15% of it's total loaded weight of 8300 lbs. As its a rear kitchen layout, the unloaded and loaded pin weight remains at the same 15%. I couldn't increase the pin weight significantly outside of putting cement blocks in the basement. It tows just fine at that pin weight...no chucking.
Most likely, a 5er pin weight will put you over the trucks GVWR. In my opinion, as long as you're under the trucks RAWR, you're good to go. Don't know what your axle ratio is, but you'd want at least 3.73 or even 4.10.
Nights camped in 2013 - 55, 2014 - 105, 2015 - 63