Nathaniel,  there is only one deep cycle battery that will fit in that
compartment under the  drivers seat.  That is the OPTIMA yellow top.
And, it will
not fit  without a few minor modifications to the compartment lid.

  Is this true even with a Wolfsburg Westfalia with the factory aux battery under the driver's seat?  I don't have swivel seats if that matters.  The PO put a starting battery in there and I need to get a new one this summer.  I don't want to go chasing after a deep cycle if none will fit.

  Thanks,
  Clint