My first question is what year is your GMC. If it has a 6 speed tranny dont expect a lot better towing with a ram. It more than likely will pull a little better but I wouldn't expect it to be night and day different.
I own a 2015 Ram 2500 with a 6.4 and 4.10's, Rams with 4.10's are hard to come by so if you are buying used it might be a real tough search. I love the way mine pulls my Openrange 367bhs, its 11,100 dry so Im over 12k camping. I live in minnesota so we dont have much for elevations. Hills it does fine will pull most hills in 4th and only drop to 3rd for the steepest at highway speeds. I dont expect to find a hill around here I cant accelerate up. 5th wheels are much better aerodynamically than bumper pulls though. So a 12k bumper pull in wind up a steep hill might be tougher. All that said if you want to stay away from a diesel you cant go wrong with a 6.4. But remember and be happy its not a diesel.
As far as maintenance, belts are easy, oil changes are easy but chrysler would like you to use synthetic oil. So oil changes are north of $70. But under a $100. I do mine every 8k like the owners manual says but that is almost all highway miles. Plugs? if you are worried about spark plugs buy a ford or a chevy because the hemi has twice as many, but they aren't due for 100k miles.
Id say drive the 3 gassers and buy the one you like the best because all of them will do about the same. However I think the ram will get the best mileage unloaded of all of them. But not by leaps and bounds.