Your two new 6s need to be "base-lined" now so you know what is "full" in the future. Also this makes sure they get a full charge right away after being on the shelf for a while and are likely undercharged a bit.
You must have an hydrometer--use the glass tube bobber type not the fat little plastic type with the sideways arrow. The SG should be 1.275 or higher. If not charge them up.
Here is more bad news! The 7300 at 13.8 cannot fully charge those 6s and they will die an early death. They need to be charged to 15 volts or so to get the SG all the way up--you need a special old time manual charger that lets the voltage keep climbing until you shut it off---NOT an "automatic" that shuts itself off when it thinks the batts are full (it is set to shut off when they are not quite really full) Of course you need a voltmeter to know when to shut off the manual charger.
So the 7355 will be good to float them on shore power after you do that full charge with the other charger. Read this and weep
http://pdf.wholesalesolar.com/battery-folder/charging_instruction_2011_2.pdf
