I'm skipping a lot of posts, because most of the smart guys have already chimed in, but there's ONE point that might not have been made that I'd like to make:
Figure out what trailer you want and will suit your needs before deciding on a truck.
If you're sticking with a travel trailer (some call it a bumper pull), if you stay under 35-ish feet long and under 8-9k GVWR you should be okay with a 1500. Might be pushing the limits but probably not illegal or unsafe. If you're going over those lengths and weights, you're likely going to want a 2500.
Unless you're getting a Scamp fifth wheel, there isn't a fifth wheel I'd put behind a modern half-ton truck, even if they claim they're 'half-ton towable'. The pin weight alone puts you over the average payload of most half-ton trucks, so forget about any passengers or gear in the truck. If you're looking at your average fifth wheel, you're going to be well-served to at least get a 2500, the 6.4 gasser is no slouch and gives you more payload because it's lighter than the diesel.
If you decide that an average camper isn't going to cut it for you, and you need to have all the luxuries and bells and whistles of a heavy 35-40' fifth wheel, you're going to want a 3500 SRW, the 6.4 will handle the job but if you plan to be on the road a lot then you may want the diesel for torque.
If you decide you have to have a top-of-the-line rig, 40+ foot long with all the bells and whistles, whether it be just a fifth wheel or a big, tall-sided toyhauler, you're getting into 3500 DRW or bigger territory (the Ford F450 is a popular choice for these rigs). These rigs come with pin weights approaching or exceeding 3k lbs that lesser trucks just can't handle, and the stability of the extra tires in the back of your truck is worth its weight in gold.
This is my two cents, take it or leave it.