I have a 2012 4X4 Ram with Hemi 5.7, factory trailer brakes much as you describe. I towed my 31 Ft. Excella from Ft. Worth, TX to Wisconsin and back last July. Lots of hills, no mountains, 11 MPG on plus gas. I did go to a Hensley hitch upon my return and it tows alot better. Hitching up to the Hensley is a pain. I'm hoping to get better at it.
The Hemi has lots of power (395HP), and handled the load well. I have purchased some KYB Mono shocks. They handled much better on my previous Ram truck with the heavy load.
The diesel is a step up in power, but I'm not sure you need that brute torque with an AS. It can make for a rough ride for the trailer and the cost of diesel fuel just scares me off. A side note: my hemi got 18 MPG the first tank of gas and is still consistently 16 - 18 MPG at highway speeds with no trailer of course.