Given your batteries.. Hook those GC-2's up solid and go with a Progressive Industries 9280 Dongle optional, Or a 9180 with the optional charge wizard.
The Parallex 7300 is a single stage converter.. Another name for that is power supply, it has fairly decent regulation but this means it will basically take a while to charge and will never, ever, finish the job if it is set right.
The PDI 9200 however.... Is a 3-stage + converter, It will charge from 50 percent in around 6 hours charge fully, And treats the batteries well..
That said.. The LOWER the rate of recharge (down to about 10% of C/20) the longer they last.. The 80 amp with the batteries you have is just shy of 30% c/20 Which is the max charge rate Xantrex suggests.
