I have a Trueam dual battery isolator between the 1st and 2nd batteries. See the Club Store.
This relay / isolator that I was having problems with is just for the 2nd battery to a fuse block for the Aux. lights.
When the key is off, the lights are off. There have been many times I forgot to turn off one of the Aux. lights.
My dad loved it for deer hunting, he would just park it with the light switches on and in the morning he was good to go.
The relay and isolator serve two different purposes.
The isolator simply ensures that your primary battery doesn't get run down if/when your AUX battery does. This assumes that you don't have any accessories that are connected to the primary battery (if you do, you're circumventing the point of installing an isolator).
The relay energizes or shuts down power going to the fuse block or anything downstream from it whenever your "switch", be it key-on power or a 12v unswitched toggle switch) is turned on or off.
Keep in mind that you don't HAVE to put every accessory on the switched fuse block (downstream from the relay).
In fact, many people install BOTH a
switched fuse block (e.g., energized by the key-on or switched relay)
AND an
always hot positive bus bar (or a second fuse block) connected directly to the AUX battery or the always hot post on the relay.
This way, you can hook up accessories that you only want available when the key is on to the switched fuse block AND hook up other accessories that you
do want to use when the key is off, while retaining the benefit of the isolator (except for the few accessories you have connected to the always hot side). You just have to be sure that anything hooked up to the "always hot" side is absolutely turned off when you are done, or the AUX battery will certainly run down.