My background is in vehicle wiring.
More accurately aircraft and bespoke automotive applications, with 30+ years in the industry, and in the past 10 years a dedicated CAN-bus design and applications field.
Wiring is relatively simple, MOST wiring will work in one form or another !.... even poorly designed systems.
I am a firm believer in KISS systems (keep it simple stupid), it is easier to sort out when it goes wrong.
Electric is identical to water, it flows, it leaks, it turns on and off. The idea of wires melting and units "blowing" is correct, but for this to happen the circuit must have energy (power), simple.... no energy, no problem.
The preliminary circuit shown below will alleviate any of the above problems, if the isolator is OFF then NO ENERGY will be in the system, no wires will burn, no units will "blow".... PROVIDED there is no other supply within the system (eg) 12v being back fed from a charging item e.t.c
The engine starting supply is completely separate, not integrated with this system at all. If charging of the leisure batteries is provided from engine alternator then a split charge relay is absolutely necessary to isolate the 12v supply from the engine starting battery when the engine is stopped.
I hope this helps.
John