Before you buy another battery, unless you need one now, you might want to reconsider your dual setup.
Don't know how yours is setup, probably on the forum, but too much trouble to look up, but it sounds like you are kinda defeating the purpose of most dual battery setups.
The system in my old truck was not perfect, but in over 10 years, it never failed me, failed to crank, and I think I used one of the batteries the full 10 years.
Only replace the other once.
I was using a diode bridge, but your setup should work as well or better. And, it was a Ford, which made the wiring a little easier.
I had a standard cranking battery in the original holder. It had two wires hooked to it.
The big main cable that cranked the starter, and a charging wire from the bridge.
Actually, it had another big battery cable over to the other battery through a Ford starter solenoid so I could do a self-jump from inside the truck, but was just frosting on the cake.
The other battery, was not a deep cycle, although it could have been. I have a local battery place that sells any type battery you want, and are experts on batteries.
Plus, they are friends of mine.
They hooked me up with a standard sized big truck battery that was awesome. He set two identical batteries down in front of me.
I picked up the name brand one, good heavy battery.
Then I tried to pick up the other one.
Took me two tries.
Said the standard name brand one had about 35 or so thick plates of lead in it. The other one had 90 thin ones.
The standard had lots of insulation, this one was almost solid lead. Reserve capacity was off the wall.
Anyway, that battery ran everything except the starter. Radio, two way, lights, everything.
With its capacity I very seldom ran it down, but if I did, the other battery was sitting there nice and hot to crank the truck.
The only time I had a problem, was when I left the light on all weekend. The second battery was so dead that it could not fire the solenoid to crank the truck.
Probably would not have powered the ignition if it had.
Since both had heavy grounds, I just used one jumper cable from positive to positive to jump myself off.
That was when I added the cable and solenoid.
When deep cycles first came out, you could not use one for cranking. They could not handle the current.
I know of several people who burned some in less than a week.
One guy put one on a big diesel tractor, it lasted less than a day.
Deep cycles today are better, and can stand occasional cranking. Some may have no problems at all.
You might consider adding even a third battery. Leave one for cranking, the other two for the inverter and other stuff
Heavy enough cable, properly protected, will let you put a battery elsewhere than under the hood.
BTW, my battery guys used to sell Optimas, but quit when they started having problems.
They sell a different AGM now, who's name escapes me. Expensive, but worth it. Personally, after replacing many battery boxes, I would never run an unsealed battery in my offroad stuff again.
I do have a winch going in. I guess I'll have to update later. 
