Greetings & Welcome!
I’m not a huge fan of heaters that also require power for several reasons, the most important drawback for me is that they wake me up every time they cycle on or off. I also don’t like the requirement of having power to have heat. I’m frequently a cold weather camper, and I prefer portable kerosene heaters myself. They are wick type, no power needed, and will run on diesel too. You do need to keep your windows cracked for ventilation with any type of portable non vented heater.
There are also portable propane heaters, but they are reported to be unsafe to run while you sleep.
On to your electrical system… My current “build” was also the cheapest and simplest for power. I just went into a used battery shop, told them what I wanted, and they did it. For $80 they installed an isolator, a used deep cycle house battery in a nice battery box, and even hooked up a temporary switch so I could jump my starting battery with my house battery if needed. One of the best $80 I’ve ever spent! In talking to them, they also suggested low battery cut-offs for both my starting battery, and my house battery, and for $20 each I had those installed as well.
While this can all be done yourself, in my case it was cheaper and much quicker to just pay to get it done. It took them under an hour for everything! I also no longer buy new batteries, now I buy used/recycled batteries either fromm a battery recycler or a junkyard. They’ll last 5-7 years and usually under $20.
For DIY, you’ll need an isolator, and some connecters and some wire. A good source for the battery to isolator to house battery wires is a good et of heavy duty jumper cables. Connectors and the wire to activate the isolator should be available at any auto parts store, and I just use a marine type battery box that you should be able to find at Walmart. Put a high amp fuse or circuit breaker between the starting battery and the isolator, and another one between the isolator and the house battery. Then you’ll need to run an 8-10 gauge fused activation wire to your fuse box, to a fuse that is only activated when the ignition is turned on. Something like the radio or heater fuse.
Maybe this diagram can help:
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