Hi cruisin,
OK, I understand your logic, but I think it's flawed. You are arguing that the 12v battery needs to be kept topped up whilst the car isn't in use or over time will need charging seperately via 12v charger. So in order avoid this you suggest leaving a parasitic load running off the main pack.
Lets take an example to show why this is such a bad idea. You can slate me for this later if I'm wrong! I'm always happy to have my thinking proven incorrect if it actually is.
Assuming a conversion has a typical 160Ah lifepo4 battery and you have your small DC/DC on it all the time, and lets assume you have a 2A fuse on the input to protect the main battery. Now lets say that the DC/DC develops a fault over a long weekend whilst the car is parked up and starts to drag a couple of amps constantly as a result from the main battery. We'll also take it the car has been charged and the charger subsequently unplugged for the safety of the pack.
Now 2A doesn't sound much does it?, but if left for only 3 1/2 days in that state the main battery that probably cost as much as the rest of the conversion put together is probably permanently dead!
Would it not be more correct to build using a correctly sized 12v battery that could last a couple of weeks minimum with the expected parasitic loads, as current ICE designs do and have a larger, suitably sized DC/DC converter capable of maintaining the necessary running loads and topping off the 12v battery whilst the car is in use only?
That way, there is no risk to the main pack, (at least from the DC/DC converter, as for a BMS left connected.... well that's an arguament for another time
)