Get the 5th wheel prep group. My truck is a 6.4L Hemi, but it has over 3000# payload. The diesel version of my truck would have around 2400 - 2500# payload.
The 3500 SRW has an 11,700# GVWR vs the 2500s 10,000#, so payload will be right around 4000#. A buddy of mine has a twin to my truck except it is a 3500 SRW diesel.
APT's info is a bit out dated on the Ram 2500s. They used to have lower GVWR and payload than they do now. The frame went from 35,000 psi steel to 50,000 psi steel in the 2014 2500s (3500s got that frame in 2013). The specs on the 2015s will be almost identical to the 2014s.