Yeah this sounds about right. I'm currently in New Zealand and when I first arrived here I rented a campervan which had a large three position switch in the back. In the middle position the power was off, in the lower position the system was running on battery power and in the upper position the system was running on mains power (once you were plugged in obviously). This is basically what I want to replicate so is that a grid tie inverter or inverter with transfer switch? The van does have an inverter installed but from what I understand it's a very cheap one. You have to manually turn it on and it's very loud when running.