I scanned through the posts so forgive me if I repeat something.
I agree with the AISIN and 4:10 for your application.
I’ve had 3) 3500 srw and went with the Aisin on all 3 and don’t regret it. You also get the HO motor with the far better trans. 385hp/910ft lb compared to 360/800. (I think the HP may be more)
Get the 5th wheel prep. Well worth the $440. The BW RVK3600 works great.
I wish I got 20 mpg in my srw SB 3:42 truck. I would expect around 15 mpg with a DRW with 4:10.
Where are you located? I would look at new trucks. Huntington Beach Dodge (So Cal) has an 18 Laramie DRW 4x4 Aisin 4:10 for around $54k. Around $15k off sticker.
You will love the Ram Cummins.
