RE: Aux battery
Since there is no redly available deep cycle battery that will fit many go
with a Walmart Group 41 or Group 48 battery.  Their warranty is great if
the battery fails because of use that should have a deep cycle.

As for the gas leak (and I probably don't have to say this) DON'T START
THAT VAN UNTIL THE LEAK IS FIXED!!  That plastic through the firewall
fitting is notorious for breaking.  There should be much of a leak with the
ignition off, that line goes directly to the fuel pump.  Fuel pump off
should be no pressure.  Metal replacements are available from vendors like
Van Cafe
<
http://www.van-cafe.com/home/van/page_1130_286/fuel_line_firewall_adapter.html
>
Many eliminate that fitting, add a grommet to the hole and pass the fuel
line through the hole.