What is the heaviest 5th wheel load anyone has towed? Will the 350 crew cab both single and dually handle a load of 5250lbs on a 5th wheel?

Best place to look for 5th wheel experience is:

http://www.rv.net/forum/index.cfm/fuseaction/listings/forum/24.cfm

There are plenty of folks, with many different truck/engine combinations that should be able to answer your question(s).

Jim