I tow a 5'er that puts my 2012 Ram 2500 over it's GVW by 100-200lbs. Obviously not much at all. I prefer to use the trucks RAWR as a base. On the 12 2500 the spring pack is the only difference. Ironically the 2500 has 5 leafs (thinner)and the 3500 has 4 leafs (thicker). Even when moving to a DRW the drive train is the same. Only the rear axle and leafs are changed. I'm not suggesting you tow way over the GVW, stay under the trucks RAWR and you should be fine.
Look for dry pin weights near 1500 and you should be good. By the time you add 150lbs for the 5th hitch and load the front of the 5'er up you'll be around 20-2200lbs. Which would put you in the 10,000lb loaded 5th wheel range.
FWIW I drop 2" in the rear with 17-1800lb loaded pin weight. Guessing you'll need air bags for the 318HC.