It may be over simple but all I have to do is to UnPlug the light cable
between the truck and the trailer.
I only do it if I am going to be stopped for over 4 hours or more. Like
over night or taking in some event.
This will stop the drain on the truck system.
If I was to use a Diode I would have a voltage drop in the charge line
that would keep the batteries in the trailer less than a full charge.
Dean