Electrical physics 001:
The smaller the gauge the wire the higher the resistance of the line over distance thus a higher voltage drop (potential) and a reduction in the ability to flow current (amperage). In the event of over current under sized wires like to melt at best or catch on fire at worst.
I am an Electrical Engineer and I am in the process of designing a 200W solar install project for my RV at 30A max. I will be using 4 gauge wiring from the panels to the charge controller and 1/0 gauge wires from the controller to the batteries. That's just for 30A max! Why? To reduce resistance so I can maximize the panel's out put to the batteries.
Let's get to being relevant to this topic. Running a 12 gauge wire from your battery through 6/7 pole connector (I also favor the 7 pole but that's just a preference. Not a requirement as they tend to be more of a standard with RV's so therefor easier to use) and it will maintain the battery but by no means charge it. At best you are going to get a 60-70% charge no matter how long you drive it meaning you have a 10-20% charge you can use without significantly reducing the life of the battery.
Do the same with a 8 gauge wire and what would the results be? About 10% more. Maybe. Not much but it's still a gain and all you had to is up the wire size. Now you have the added peace of mind knowing you won't over heat your wiring as well. Not saying you would with your #12 but you have significantly reduced the likely hood.
At the end of the day though relying on your truck to charge your RV battery is not going to cut it. Deep cycle batteries life curves drop off quickly if you deplete them more than 50%. The only way to be able to rely on the battery to last more than a year is to fully charge it before you start using it. Daily. There are lots of ways to accomplish this but it all starts with a quality charger. One that you can program to the type of battery you have (AGM, Gel, Flooded), has a temperature sense line, and has access to enough voltage to properly charge the battery. The power source can be a generator, your A/C wall socket, a solar setup, etc.
Larger diameter wire = faster recharging.
Shorter wire = faster recharging. Since you can't really make your truck or RV shorter (realistically) most people tend to opt for bigger wiring.
Again, this doesn't mean you can't use smaller wiring. A lot of people do but then they pay for it in the end by replacing a battery every 1-2 years because they constantly drained the battery below 50%. Even cheap batteries aren't cheap and buying a bigger battery just means a bigger cost to replace in that 1-2 year cycle. Properly charging a battery will net you a 3-5 year average life span and it is not unheard of to get 8+ years if you are doing it right.
So now you have heard it from an Electrical Engineer. Here's my disclosure. YMMV and if you aren't sure use your favorite form of research tool, seek professional guidance from a certified electrician, etc. I am merely stating what the laws of physics (as we know them) dictate, practical experience, and my own independent research beyond what I learned in school. My senior design project was an actual solar power installation at a Navajo hogan we designed from the ground up (yes it worked). I also did a solar panel setup for my pop up and was fully charging the battery each day off one 100W solar panel. Never once did I use a generator or my vehicle to charge the battery while camping. Now having said that in no way implies I am an expert. It just means I did my home work, came up with a design, and then put all the pieces together just to prove that I could do it myself.
It's your truck so do it your way. There is no wrong answer as long as you are happy with the results. Oh, and you can use a solar panel to charge your battery while driving if you secure the panel to the roof of the trailer as long as it isn't in the shade.
eace:
