Your current trailer is 12k lbs and you say you may have a heavier trailer some time in the future.
The diesel makes sense/cents in your situation over any of the high hp smallblock gas engines at those weights.
My RV trailers weighs 11200-11400 depending how long the trip will be. I also pull a 16K GN tri axle GN stock trailer.
My '03 2500 Dodge /Cummins NV5600 3.73 axle pulls both fine and with 278k miles on the odo the truck has had very little out of pocket maintenance expenses since I bought it new in 1-20-'03.
Stay under 14k-15k lbs with a 350/3500 SRW GM/RAM or Ford with the right package for max payloads and you won't need a DRW truck.