With the current Ram 2500 and 3500 they share the same stronger frame and probably share most of their other components except for the engine and the transmission. Either will handle a 5th wheel travel trailer load of 15,000 lbs. or more.
For a trailer weight over 10,000 lbs. I would go with the diesel truck. The diesel provides nearly double the torque and that is what counts when accelerating into traffic, passing a slow tractor trailer rig on a grade, and when using engine braking coming down a steep grade.
With a gas engine I would not buy a truck without 4.10 or lower gears. 3.73 gears reduce the towing capacity by 20%. Check the Ram trailering guide for specifics. A 5th wheel with a weight of 11,000 lbs. can be pulled with the gas engine powered truck with the 4.10 gears quite well.
I would also check before buying that a Titan aftermarket fuel tank that replaces the stock fuel tank is available for both trucks. I bought a standard bed pickup and later learned that Titan decided not to make a replacement fuel tank so I have the 35 gallon stock tank instead of a 60 gallon plus tank and when towing my trucks MPG drops by 25% meaning I have 25% less range on a full tank of fuel - diesel in my case. As 3 out of 4 gas stations don't sell diesel in the west than means more trip planning and stopping sooner to get diesel than I would if I had a truck with a gas engine.
Don't think that you need DRW for your 5th wheel as even the heaviest travel type at 13,500 lbs. can be towed with a DRW truck.