The only battery that will fit is a Group 41 or 42, the 42 being
smaller.  No one seems to have found a sealed/gell/gas mat type battery
that will fit.  But do you really need a deep cycle battery?  I just
replaced my aux battery (yesterday in fact).  I went with what has
served me well over the years, a standard starting battery.  And I have
quite a bit of 'stuff' that I run off the aux battery, sometimes for up
to a week. It works fine.
If you are going to be running a heavy load off the aux battery with the
engine off, and doing this on a daily basis then consider the extra
expense of a deep cycle battery.  If you are only going to be drawing
down the aux battery on those few occasions that you are camping (I
assume you are not full-timing in the Westy) then save some money and go
with the standard battery, about a third or less the cost of the fancy
stuff.  If you have to replace it every couple of years, your still
ahead.
Costco used to carry the Group 41, picked up two for my '85 there in the
spring of 2001.  When I checked recently, no group 41's on the rack.  I
didn't ask if I could special order them, went to the local battery
shop, swapped lies about Westfalias (the shop manager drive a nice '75)
and had him pop one in, five year warranty.