I have the '12 3500 DRW with the regular 6 spd, works great with our 42' 5er. One of our ambulances at work is a '13 4500/ aisin transmission, works well. The only difference I can tell is shift point are a little higher in rpm. May be the rear gears too. Can't go wrong with either. The cummins is a beast.
Edit: we obviously don't tow with the ambulance, but it performs well for how we use it.
2016 Forester 3171DS Class C Motorhome
2016 Jeep Wrangler Unlimited Sport - Toad
1993 Mustang 5.0 Notch 5-speed