Towing with Auto vs. Manual opinions. Looking for a TV

My dad ran the 99 F350 4x4 with the 7.3 PowerStroke and the 6 speed manual over duallys. When he sold the truck due to illness, the rancher who bought it took it home to MT and called back to Idaho and said he'd never need another truck or any more pulling power. He planned on using it to haul cattle to KC for sale on a regular basis. He had been looking for months for a truck like that.

2005 GMC Sierra 2500HD 4x4 Ext. Cab Short Box 6.0L
1998 Jayco Designer 2930RKS