personally I would stick with a 350... a 350 can be purchased as a dually or singled, where the 450 is always a dually.
the 450 offers nothing more than a 350 dually except a 1000lb more weight capacity and somewhat lower mileage.... not a good enough trade off in my opinion, especially when the 350 has everything it needs to pull an RV.... if it was being purchased as a commercial vehicle and pulling maximum loads, it may be different.
one thing to look at is the transmission... different brands and different years, the 450's can sometimes be found with heavier transmissions in them for commercial duty applications, where sometimes these heavier transmissions arent available in the 350's