I think the 2013 would be a better choice. I have a 2012 which I really like. It has the Max Tow package. I think the Aisin transmission is available in 2013. The 2013 uses DEF but should have better fuel mileage. Other considerations would be how the trucks are equipped. In my opinion the Max Tow package is necessary as are any other options that improve towability. I would love to get a 2016 or 2017 but money is not falling off our tree right now plus I do like my present truck. I tow our 36' MS plus the RZR on a trailer behind with no problems.
Slow six
You are probably unloading some of the pin weight by double towing.
My 36 Elite has over 4000 lb on the pin.
That said:
So no matter what 3500 you get Kiltedpig you will need air bags..
You may want to weight the MS before hand to get a better idea of the weight you will have in the bed of the truck.
Then decide what truck to get
Kevin
No air bags and 4,500# pin in this pic. 11 HO Dually. I did add bags and Bilstein shocks to control some of the horrible road conditions. Only ran 30psi loaded.
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2015 RAM LongHorn Dually Air ride Aisin 4:10's