Joel Rodney Siemion

I have a 2017 Transit connect with the long wheel base. I am interested in using the cargo area as a mini RV. I have not seen an answer as to where is best to install a secondary deep cycle battery. I was told that the storage compartment under the passenger seat was supposedly designed for this? I do not believe that is vented so only an AGM should be used? I read there is a spot for a cable run in the firewall already? Any help or suggestions are appreciated-Joel.