In my opinion the best bang for the buck is to use the standard group 41 starting battery and if possible take advantage of the warranty that comes with these. If you need more capacity you will have to give up space someplace else. 
For a charge control relay there are a lot of options but even more important is to provide a good charge source. I now typically run a dedicated #8 charge line from the alternator to the relay of choice and then to the battery. This approach works out well. That charge line needs to support charging the battery along with any loads that will be connected such as the refrigerator along with your entertainment system and anything with minimal voltage drop in order for the battery to get a good charge. 

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of Andre Fiedler
Sent: Wednesday, June 1, 2016 12:33 AM
To: vanagon@GERRY.VANAGON.COM
Subject: Auxiliary battery recommendations

The auxiliary battery in my Westy no longer seems to be holding a charge. Any suggestions as to what is the largest deep cycle battery currently on the market that I can get that will fit in the spot behind/below the drivers seat? Also any suggestions as to a good battery combiner/separator?
Thank you.
Andre Fiedler
89 Westy