I'm still confused as to what the GVW of the OP's 5er is?? Mine has a GVW of 16,900#, loaded heavy for camping, wife, myself and two boxers is 16,050# with a pin weight of a little over 3300#.....I've towed this 5er with an 07.5 Chevy Dually, a 2010 Ram 3500 Dually, a 2012 Ram Dually, (both with 4:10),and looking forward to towing my with my 2013 in my sig....Worse mpg I EVER got towing with the dually through the mountains of WV, TN, NC, SC and/or VA is 9.5 mpg, best has been a little over 11 mpg....Dually is also my daily driver...been my daily driver since 05'.
If OP's 5er DOES have a GVWR of 16,500#, I'd look for the 3500 SRW with the highest GVWR, a 4:10 or 3:73 with the highest RAWR...IF they REALLY, REALLY want/need a SRW truck
Dually in my sig. has the Aisin tranny with 4:10...has a GVWR of 14,000#, a RAWR of 9750# and a GCWR of 37,500#, and a payload of 5660#, but that's just keeping you within GVWR, I could easily go with a 5,000# pin weight and not get near my trucks RAWR of 9750#...a bit of overkill now for my 5er, but, who knows, I might want to go heavier one day:)
Believe a new 3500 SRW truck would probably work, but I'd do the weights, and I'd use my units GVWR and would figure a minimum of 20% for pin weight.....that's MIN. for that 5er.