Just run a separate charging circuit to converter. A simple way is to take a 50' extension cord. Cut it in half. Plug it together with some play at hitch. A few zip ties. Then run one end to truck battery, other to converter. Unplug it when you turn off truck. If you need larger wire Buy two jumper cables. Cut off the ends and buy a pair of winch type plugs for the hitch junction. If you want to get fancy run the wire in standard wire loom cover then feed it under truck, zip tie as required. Put a breaker and switch under hood to cut power when not needed.
