Wiring a DC to DC 20 Amp charger for a small travel trailer.

I’d like to wire a DC to DC charger adjacent to the battery box on my small travel trailer. The wiring is pretty straightforward. Positive and negative 8 gauge wires running from the vehicle’s battery to an Anderson outlet at the rear bumper. My question is about the “ignition” wire that would run from the under hood fuse box to the outlet at the bumper. The purpose is to prevent charging the auxiliary battery if the vehicle isn’t running. They sell a fuse with a pigtail that would go in an empty fuse slot in the fuse box. I’m not sure what Amp fuse I should use and what’s an appropriate empty slot that wouldn’t be energized unless the vehicle is running? Any suggestions would really be appreciated.
Thanks

 

I'm about to put an auxiliary 12 V power outlet on my passenger side. I want it to be powered when the key is "ON", only. I'm going to run my power feeder from the battery, through an SAE disconnect and fuse to a 20 amp rated relay. I'll use an Add-A-Fuse from circuit 13, the seat heater feed, to energize the relay coil when the key is on.

A similar set up would work for you. The relay coil takes very little current. Just fuse it for the wire size between the added fuse and the coil.

 

If you are wanting to keep a secondary battery charged, without the risk of drawing down your primary battery, I recommend a solution using the Redarc controller used in my dual battery setup as described in the link below

https://www.4runners.com/threads/cowboys-dual-battery-install.21998/

 

$1265 vs $50?