FWIW, in 2013 I was in the market for medium duty and was considering 450 and 550. Different requirement than RV'ng, but what I found was that for just a bit more (I think 1500 or so on a list price basis)I got the F550 vs. the 450 and the slightly higher capacities, etc. 4WD, full Crew Cab configuration, cab room like you wouldn't believe.
Have not had any problems in the time since with the 550. Not many miles, but pretty severe service (plowing and hauling). Very pleasant to drive actually and I really like the ability to hook up just about any trailer (the 30' Beaver with the D6 is a bit much) and pull away without problem.
The turning radius on the 2013 F550 is significantly improved from the 2006 F350 that I sold some time back.....I think they've modified the steering geometry on all the 350/450/550 trucks in the interim, but it might be worth confirming if you get into a 350.