Charging a deep cycle battery with lighter guage wire

Your idea is practical and I would suggest useing 10ga wire. Applied to low voltage dc current,all 10ga wire is not equal. The more strands in the 10ga conductor the better to carry current without heating and subsequently leading to increased voltage drop. Insert a 30 amp inline fuse or circuit breaker as near truck battery and alternator as practical. As has been suggested,you can go through 7 way prefably center #7 pin or use #4 if #7 is in use already. A word of caution if you tow other trailers or tow this trailer with other trucks. #7 is normaly reserved for back-up lights on trailer. #4 is for 12 volt and may be in use so if you use #4 and it's less than 10ga,confirm that #4 is 12 volt to trailer battery then change to 10ga conductor from truck. Make certain #4 from 7 way to trailer battery is 10ga or larger. Unless there is an isolator inline or 7 way unplugged while useing trailer battery,the truck battery will be discharged at same rate as trailer's. Confirm that #1 is white ground at least 10ga. If #1 is smaller than 10ga,add ( double lug )a 10ga on both sides. Route truck side to nearest place on chassis it can be well grounded to. Attach trailer side to close by trailer frame then continue a 10ga to trailer battery negitive cable. This arrangement will carry up to 30 amps to charge battery. While it is true that if trailer battery is near discharged and truck battery is full charged,the amp flow to trailer battery will only be 15 to 20 amps and drop as battery charges up,that's not nessarly a bad thing. If the alternator could "see"only the dead battery it would deliver 50 to 100 amps then blow the 30 amp fuse before melting wire.(Bet some were wondering why I suggested a fuse). If you don't normaly drive long enough to bring trailer to full charge,you can switch fuse to fusable link and increase wire size to accomindate alternators full output along with a battery switch that lets alternator "see"only trailer and fast charge when you choose. The wire you need is welding lead that will cost some coin which makes a gen set look like better option.

The longer the smaller wire the more voltage is lost before it ever gets to the battery. You could be down 2 volts or more. When you need 14+ volts to properly charge a battery, having only say 12.5 at the battery gives you very little charging. A bigger wire has less line loss and can pass more amps at the same time without getting hot. Wire getting hot means too much resistance to the flow of electricity means less voltage at the end again.
Auto generators (alternators) sense the charge level of the battery and reduce their output in volts and amps as the battery comes up on charge. That's why they start out at 14+ volts on a "dead" battery and drop to 13 something after a while. If you try to charge your RV battery with jumper cables by clamping to the posts of the car battery the alternator will sense the voltage of the car battery and lower its charging capability "thinking " the battery is coming up to full charge. To do a good quick charge with the car alternator remove one of the leads to the battery in your car and the alternator will only see the dead battery and put in a good heavy charge until it comes up to voltage. You can actually hear the alternator load down if you remove the one lead while charging both batteries. The engine if at idle will slow down with the extra load. If you're going to drive every day then the wire from the 7 pin connector will in all probability bring your battery up to a usable level but never fully charge it. If it never gets fully charged it will fail sooner than planned from sulfation of the plates. As was mentioned - YOU NEED THE BATTERY IN THE TRAILER FOR THE EMERGENCY BRAKES TO WORK ON THE BREAKAWAY SWITCH

Run 8ga as it is not that much larger in diameter but provides double the cross section area of a 12ga wire and so delivers more current to your camper battery.

I was thinking about running it through the trailer lights on the connection, but will it really not charge off of the loom wire? I was thinking about using some old jumper cable wire, but isn't that aluminium?

Use heavy duty jumper cables if you must charge the RV battery from the truck. If pulling wire go with #4, as #12 will do about nothing.

Truth is it will all be a PITN and very slow leading to an early battery death from sulfation.

If parked in the sun look at getting 100 + watts of solar or a dreaded generator with a decent portable charger.

mbopp wrote:
What truck / trailer combination do you have? 7-pin trailer connections have a pin for a charge line and it can be wired to the battery and charged off the truck.

X2. You also need that battery to be in the trailer if you have electric brakes with the breakaway switch. Then if you accidentally disconnect - the battery will be used to apply the brakes.

I agree with mbopp - 7 pin connections have a charge line that uses the same wire gauge as the rest of the loom and is used to keep the trailer battery charged.

What truck / trailer combination do you have? 7-pin trailer connections have a pin for a charge line and it can be wired to the battery and charged off the truck.

I would opt for heavier gauge wire, you'll get a lot more into the battery with a bigger wire.