2. Find a controller similar to a solar controller. Is there a solar controller that will accept 13.6 v input.
This place sells what you're looking for.
DC Input Battery Chargers for Lead Acid and SLA Batteries, for charging batteries from vehicles and other low voltage sources, 6 Volt, 12 Volt, 24 Volt, 36 Volt, 48 Volt, 12V
It's nice to see the photo I took of my S's fuse box being used to help! I shunted the relay controlling the 12V outlet (see my reposted photos) and leave my Dometic CF-35 refrigerator plugged in when traveling. Works great.