Maybe this will help - -
http://www.ford.com/trucks/superduty/specifications/towing/
This is for the 2014 year but 11s on have not changed. The lower block is for 5th wheel trailers and the upper is for conventional towing trailers.
Be sure to follow across the line to find DRW for 350 and 450. All cab configurations are shown.
As was said before, , , the 350 is more than enough truck for a 5th wheel RV. For a couple more thousand bucks and some lost fuel mileage you can have the added assurance of the wide front axle and HUGE brakes.
Be sure to ask the difference in warranty between these trucks as there is some. The F450 is made to be on the road loaded constantly. Hot Shot haulers, ambulances and wreckers would fall into this category. That is why only a 430 ratio is available. The truck is designed to be working most of the time. The 430 ratio takes some load off the diesel and transmission.
The F350 is more for heavy duty part time work as on a farm or business or towing RVs around the country!! This truck is given a 373 ratio to help with fuel as it is not meant to pull/haul all the time.
Because the trucks are designed for different work, Ford has different warranty for each. After you melt down all the small print, , both are covered for 100k miles just in different ways. None of this would mean much on a used purchase though.