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
