Even if you have a lighter toyhauler, a dually handles better in bad weather (wind, rain) as well as in curves, the mountains, etc.
The weights of an RV are the weights as they leave the factory so you add batteries, water, gas (in the on board gas station), clothes, food, dishes, motorcycle(s), tools, equipment and supplies, etc. bumps your weight up way more than most people might think. I have a 2014 F-350 dually, FX-4, Longbed, Lariet, crewcab and I was still over weight by 1,020 lbs.
The F-450 is the same motor, tranny, rear end but has a bit wider stance, beefed up suspension and surprisingly, has a tighter turning radius than the F-350. That can come in real handy in some of the state and national parks or older RV parks where things are a little tighter.
All three brands.....the later model trucks have a heavier weight and tow capacity than they did 8-10 yrs. ago. The pickup boxes are generally a little bigger, also.