I have the big gas engine in my truck and have been very satisfied with the performance towing my 5er which comes in around 12,000# loaded ready to camp.
A 5er that is ~12,000# dry will be ~14,000# loaded, and that might be OK for the Ram 6.4L Hemi/4.10 or Ford 6.2L/4.30 combo if you're not in a huge hurry or are really are against owning a diesel.
The Ram 6.4L Hemi/4.10 is rated at 15,000 to 16,000# max tow and 22,500# GCWR depending on how it's configured. Mine is rated for 15,470# towing and 22,500# GCW.
The Ford F350 6.2L gas engine 4.30 is rated for very similar numbers.