I've seen three different setups.  One was a Curt flip to the side type ball mounted behind the 5th wheel.  Another looked like a B&W flip over ball type hitch mounted to a piece of plate and welded to the truck frame behind the 5th wheel plate.  The third was a regular non turnover type welded in between the frame rails on the bottom flange so it was out of the way of the 5th wheel plate.  Two of the three were welded to the truck, I don't remember about the flip to the side type.  Here are some links to the style I'm talking about.

1
  http://www.jegs.com/i/Curt/229/61052/10002/-1?CAWELAID=1710587171&catargetid=1784156093&cadevice=c&&cagpspn=pla&gclid=CO7c4KzGq7gCFeoWMgodVUAAew

2
http://www.valleyvet.com/ct_detail.html?pgguid=30e0720b-7b6a-11d5-a192-00b0d0204ae5&ccd=IFM003&utm_source=google&utm_medium=cpc&utm_campaign=F%2BCat%2BLivestock%2BEquip%2BAnd%2BSupp%2B(6000)%2BPLA&mr:trackingCode=3954C464-3C81-E211-BA78-001B21631C34&mr:referralID=NA&mr:adType=pla&mr:ad=14740580683&mr:keyword=&mr:match=&mr:filter=34501482403&gclid=CKvMnMXGq7gCFY9AMgodQHIAHw

3
http://www.walmart.com/ip/21316224?wmlspartner=wlpa&adid=22222222227000000000&wl0=&wl1=g&wl2=c&wl3=21486607510&wl4=&wl5=pla&wl6=19880599990&veh=sem