Trailer wiring help

X2 on using an Anderson Power Plug for charging a battery like this. The charge wire in the typical 7 pin RV trailer connector is too small to be of any use. I used an APP connector to connect our camper to the truck. Can get them from Grainger, powerwerks, or West Mtn Radio to name a few vendors. I used 6ga. duplex wire from West Marine.

I'd also give some thought to installing a VSR/ACR between the batteries. A "Voltage Sensing Relay" or an "Automatic Charge Relay" make sure that the starting battery gets recharged first, and then connects the trailer battery for charge. When the charge source stops for any reason the ACR/VSR disconnects so that you don't draw down the starting battery. Del City linked above carries the excellent BEP Marine product under "Specialty Relays". Unless the trailer is going to have solar AND you want that solar to charge the tow rig's battery you want the "Single Sense" version.