Been looking for simple drawing for you but hate to pass along something that is different than what you really have their...
This is probably the most simplest dual battery setup...
This setup when engaged will discharge both batteries from your accessories being used.
Where things are different is where the +12VDC from the ignition switch or manual over-ride comes from to operate your solenoid.
The smart systems control the +12VDC going to the solenoid and switches it in when needed. The grand idea is to NEVER run down your truck start battery when you start pulling current with your extra accessories. The smart solenoid comes into play where the truck alternator is running the system and is producing charge DC voltages (i.e. 14.4VDC) and then tells the solenoid to combine the two system so that the auxiliary batteries get the charge DC voltage the same time the truck start battery is being charged.
This is what you really want to happen and just have beefed up batteries on the camping side to handle your bigger load.
There is also another issue with just paralleling the camper batteries to the truck alternator system is if you exceed the current drain rating of the alternator it may destroy your truck alternator. You don't want that to happen... You want your camping battery isolated from the truck start system when pulling all of your high currents like when using a high wattage Inverter etc...
The diagram above is very prone to blowing your alternator if high enough DC current is pulled from it when you have the truck system running from the motor.
Hope this makes some sense in what you are wanting to do.
This is another diagram showing a simple smart mode solenoid relay installed. Ignore the boat motor shown in the pictorial. In our discussion this would be a truck alternator instead.
MEX is the expert here on vehicle alternators and high current producing system...
I personally would rather run down my camping batteries to their 50% charge rate and not bother the truck battery system at all...
All of this is really intended to determine what kind of dual system you have there. Your description of events sort of points to a smart mode system.
Roy Ken
