Newer diesels don't smell as much and if they smoke then there is a problem. When we were looking at our first diesel, the first thing I said to try and get an edge on the salesman is that diesels are loud and they stink. Without missing a beat (he'd probably heard this so many times) he said, "So keep your windows up".
These days with the modern diesel motors even DW will like the ride, the power and the mpgs when you get the diesel and they aren't that loud, stinky or smokey. We are on our second diesel cuz we keep getting bigger trailers.
MPGs will depend on the load, the size of the rig (more wind resistance) and how you drive but it will definitely be better than a gasser. I'll never buy another gasser tow vehicle again.
As far as fivers go, you should do very well with a diesel and less than 6,000 lbs, but buy the fifth wheel hitch to that is over the max towing fifth wheel rating of the truck so you aren't buying hitches per your load. There's nothing wrong with having a higher rated truck and hitch when towing a lighter trailer, but at least when the need arises you can tow something bigger.