Unfortunately, that's pretty typical of most stock converter/chargers and wiring. If most of your camping is on shore power and seasonal, and you're just looking to improve the efficiency of your current charging system, the first step should be to check the output voltage directly at the converter. A $10 multimeter should do the trick (if you don't currently have one).
More than likely, your converter/charger is outputting 14-14.4v. If that is they case, then you're probably just looking at an under gauge wiring issue. Doing a quick internet search for "ampacity awg chart" should provide you with the information you need.
The issue being that D/C output voltage drops over distance.. depending on the size of wire it is traveling through. So while your converter/charge may be providing the voltage for a semi decent charge.. it's not all getting to your batteries.
So in the chart.. what you are looking for is the total physical distance (doubled) that the current must travel to your batteries. In the chart, it will tell you the minimum wire gauge required to deliver the full output from your converter/charger. Albiet, in D/C wiring.. bigger is always better. If you can afford to bump it up a gauge or two I would certainly recommend it. You also want to be conscientious of the types and size of connectors you are using in your charging loop.
This would be the least expensive way to take better advantage of your nice little 6v'rs you've got there.
In a nutshell.. it does not matter how good of a charger you plug in... if the juice can't get to your batteries then it is all for naught.
If you rely on your batteries for dry camping situations.. that is a WHOLE nuther animal.