Now would be a good time to install a battery disconnect if you haven't already. I recommend a Blue Seas 6006m for a two 6-volt battery bank or the 9001e with two 12-volt batteries.
You said you were adding a second battery so I assume you want two 12-volts. Remember to match the new battery with the old.
And as others have said, your WFCO 8955 will do the job.
DW, 3 Kids and our Goldens
2012 Shamrock 233S
2008 Toyota Sequoia 5.7L 4WD