12V electrical question

Your converter is always connected to the battery, never ever is it not connected unless you physically remove the wires from the battery
The wire from the isolator is just another charge source of another battery as far as the DC power is concerned
The isolator keeps the house batteries from draining the engine battery and let's the alternator charge all batteries
No relay is needed between the converter and the rest of the system

There's no problems whatsoever with having it all hooked together. It won't harm the converter any; it just basically shuts off if the voltage it senses exceeds its setpoint, such as when the alternator is supplying power.

It would be the same as if you had a solar installation. Rather than the power coming form solar cells, it comes from the alternator; however, it's all the same so far as the converter is concerned. Similarly, you could also hook up a second converter or a 12V battery charger if you for some reason wanted or needed to.

It probably goes without saying, but this is emphatically not the case for AC circuits. Different AC sources can be out of phase with each other, and cause tremendous currents and associated very bad effects if connected together in parallel. That's one reason why it's absolutely essential to have some sort of a transfer switching arrangement between a generator and shore power connection. (Generators that can be paired together, such as Honda inverter generators, have special circuitry built into them to synchronize their outputs.)

Thanks for the input greatly appreciated. What I have read indicates that this is right, but it does not seem right. Without your guidance I would likely add a relay to keep the converter separate.

I have used the 12V side but left the wire from the isolater unhooked.
Many thanks Jim SW FL

Thanks for the input greatly appreciated. What I have read indicates that this is right, but it does not seem right. Without your guidance I would likely add a relay to keep the converter separate.

I have used the 12V side but left the wire from the isolater unhooked.
Many thanks Jim SW FL

For the first "Sea trials", leave the converter unpowered and start the vehicle. You should have measured the standing battery voltage before starting. Observe voltage with alternator running. Power up converter and again observe voltage at the battery. Turn off vehicle and again observe voltage.

It's nice if you can do this with a meter that has event memory but most of us are not blessed with a meter with that function. Typically, both power sources should behave as stated above. No worries.

With shore/gen power and you starting the truck engine, the isolator will close and tie the chassis (truck) battery to the rest of the system. The voltage regulator for the alternator will most likely keep the alternator from charging and the converter will continue to charge, but it is possible that the engine alternator takes the full load and the converter will stop charging.

In the panel building world, with 2 power supplies that are paralleled and are not designed to be paralleled, one will take the full load while the other loafs along doing next to nothing. The small difference in 'desired' charging voltage will cause the lower voltage unit (even though both are theoretically the same voltage) to simply lower and stop the output.

I've had my truck running and connected to my trailer while still on shore power. Same thing as you've drawn. It is similar to jump starting a vehicle (battery charger + alternator, or alternator + alternator).

Having the converter output and truck battery/alternator connected together does not cause any problems when both are operating. It's commonly done in most motorized RV's in fact. The worst effect, and a harmless one at that, is that the converter "sees" the alternator output as being a fully charged battery and backs down it's own charge rate. Since the alternator is still charging both batteries at that point, everything is okay. There is no backfeeding as such, since both outputs are diode protected.