If it has a ball,  it  should be  at the bottom end of the  steel pipe..   and held in with a tripod type plastic retainer.

Last year,  I put on a  flat bed (circle D)  on our   3500,  and    instead of   mounting the  typical  filler neck under the  bed like most... and  cuss that it doesn't work,   I  welded  a  nice  filler  gusset  on the headache rack,  and  put in a  nice  SS  grommet/flange that  holds  the  OEM  filler neck.     Of  course,   I needed to     cut a  hole  in the  deck  where it  drops  down,  but  it's   tucked in  to the extreme  corner of the  flat bed.    4 feet of  multipurpose   hose   connects that  with  2  bends,   about   2 feet of  flat run,  and  finally the  tank.

Fills  like a  charm,  no matter  how fast.

Check your   vent return first!    it might be choked down  for  'safety reasons'... (roll over  device  etc)

       Usually,     'burping'  is  more  not letting  air out of the tank,   rather that  putting in liquid too fast.