Dually vs Straight

You have a 14,400# GVWR 5vr......well within the capabilities of that 2015 SRW.

I towed FT a 14K 5vr with my 2007 SRW.
No white knuckles, no feeling of being 'pushed', no wondering what IF rear tire has a flat (trucks don't have dually front axles).

A SRW is a fine towing platform. As is a SUV, 1/2 ton, 3/4 ton etc.
PROVIDED one stays within the trucks capabilities (axle ratings, tire load ratings, actual payload etc.)

You have the 5vr.......so you should know what the weights are (loaded/pin weight etc.)
Run the numbers.......if within trucks ratings and you want the SRW go for it.

Seriously why would you want to go backwards?

We went the opposite direction. Started pulling our Winslow with a 2012 GMC 3500 single rear wheel with the standard (6' 8" bed). It was great on smooth roads but we were pushing the load carrying capacity of the truck, especially on the rear axle. We had a pin weight of almost 3k pounds. The problems came on the rough, uneven stuff. We would get quite a bit of bucking, chucking, whatever you want to call it, even with a Glide Ride pin box. It was bad enough on occasion to scare the wife. Other than that, stopping, accelerating, stability, etc. were all good. I would suggest checking the weight that will be on the rear axle. We changed to the dually and all is good. It pulls and drives very similar to the 2012 but the ride is so, and I mean so much better. The big difference is the 2015's are so much quieter inside.

Another issue to consider is the turning radius you would have with the shorter bed unless you get or have a slider hitch. Our Winslow has the indented corners on the front cap to allow for a sharper angle but I can see your HR does not.

The 2012 had the DEF as does my new one, of course. I consider it a non issue. I am not aware of any problems with the DEF system on the new models. The drivetrain is basically the same as the 2011. I can get 6 to 8K miles before I need a fill up. I usually carry a 2 1/2 gallon box of the stuff from W-Mart when we travel.

Can't speak to DEF luckily my Duramax is pre DEF.

I have pulled my 38ft 5th wheel with 4 different trucks... two were duallys and two were SRW. The duallys always felt more stable.

Both of the SRWs felt as if the front of the 5th wheel was pushing the rear of the truck when cornering, kind of a squishy feel. Tail wag the dog... The duallys never felt that way.

While towing the 5th wheel, I've had one rear tire blow on the dually after hitting a rock while crossing a bridge in St Louis. No effect to the stability, I was able to slow down and get across the bridge and pull off to change the tire. At 55MPH I wonder how the SRW would have reacted with a blown rear tire on that bridge...can you say Davy Jone's Locker. Just kidding it probably would have been ok as long as I didn't panic and slam on the brakes.

It never even crossed my mind to buy a SRW when shopping for my latest truck. I like the stability of the dually.

By the way all four trucks had long beds. We carry too much stuff to go with a shorter bed.