I know this is a Class C forum, but this applies across models. My trailer (see below) had 6 guage wire run from the battery, all the way to the back for the power distribution panel - probably 20-30' of pretty narrow guage wire.
A few things about batteries - for a float charge you need to be getting 13.4 V @ the terminals....that should give you about a 1AH charge rate (VERY SLOW). So, lets say my factory 2 stage converter is doing a good job and putting out 13.6V for trickle charge. The issue now is cable loss.
I used http://www.nooutage.com/vdrop.htm
to calcualte how much voltage drop I get across that long cable run. Assuming 25' of 6awg and only 1 amps of draw, I don't do well with .2% voltage drop. That sounds like a small number, but it means I'm only putting 11V on my batteries. IE: My batteries had NEVER had a good charge off the converter
Even if my converter was putting out a much better 14V, at .1% drop over that cable I'm only getting 12.6V at the terminals - IE: getting what a fully charged battery has - and as I'm not putting more voltage on it than the battery has itself, I am getting no charge!
No matter how much I ran the generator, I could not make it through more than a night with no heater!!
My fix was replacing the 2 stage with a good 3 stage converter, and relocating the converted to the front of the trailer. I ran larger gauge wire back to the power distribution panel and a nice 4G wire the ~5 feet from the new install location to the batteries. I also replaced the now shot 2xGroup 24's with 2xGolf Carts. I'll try to get around to doing a proper write up on this mod as I suspect MANY more people out there need to consider doing something similar.
It wasn't a simple job - and a fair understanding of cabling/wiring is recommended. However, now I can boondock with confidence.
To really see how your batteries are doing, get an inexpensive 12V meter and a hydrometer. Should be out less than $30 for both and be able to KNOW where you sit.
This is a long read - but very good info and where I started. 12V Side of Life
Then when I got serious about the mod, I got involved over at the Wind Sun Forums
. Those wind-sun guys are solar, but the concepts are the same and that is where I learned the most.