Extra Batteries and Inverters

Your connection of the 110v feed from the inverter needs to be isolated from shore power and generator power in a 100% failsafe method. The easy way is to run some outlets (Additional new or re-wired existing) that are dedicated to the inverter. When the inverter is in use, plug into them. When not, plug into the others.
Or else you can do it with an additional automatic transfer switch. There are articles online how to rig up automatic transfer switches in pairs to isolate and prioritize the power source that is feeding the coach wiring. Shore, gen, or inverter.

“The camper/house wiring in comparison seems even simpler.”

Provided you use an adequate gauge wire, yes.

Seems something like this ought to work, I'd just put it downstream of the transfer switch I must already have that switches from shore power to generator.

https://www.campingworld.com/trc-30-amp-automatic-transfer-switch-75000.html?gclsrc=aw.ds&&gclid=Cj0KCQjwmpb0BRCBARIsAG7y4zarJ02Tr9emyGwR5BU4T4pVhOl0vwp6nioybc4WFxVGu8uRpf6inc8aArNGEALw_wcB

I'm guessing one of the 2 inputs is considered the priority input and will favor it over the backup source?

https://www.doityourselfrv.com/rv-inverter-install-diy/

THis should be helpful

That was kinda generic unless I missed something, I just need to hunt a little more for a diagram on how the transfer switch does it's thing. And get my hands on the schematic for my camper. I think I have those.

I just installed a second transfer switch in my new Northstar 12 STC a few weeks ago. The camper came with a transfer switch installed to switch between shore power and generator power. If you are connected to shore power, and start the generator, after 30 seconds or so the switch disconnects the shore power and almost instantly connects the generator power. I added a second transfer switch that switches between the output from the first transfer switch (either shore power or generator power) and my inverter. The only tricky part to a transfer switch is choosing the priority for the switch. There are two input connections to a transfer switch, usually called shore and generator. The generator input is the priority, if both shore and generator are connected, the switch will always shut off shore and connect the generator. If the generator is turned off, the switch will switch back to shore.

I took the output from the first transfer switch, and connected it to the "Generator" input of my second transfer switch. I then connected my inverter to the "Shore" input. Then I connected the output to the 110V distribution panel in the power center of the camper. So now, if I'm running on the inverter and plug into shore power, the second switch disconnects from the inverter and connects to the shore power. If I'm running on the inverter and have no shore power, then start the generator, the second switch disconnects from the inverter and connects the generator.

So the priority for 110V in my camper is lowest priority inverter, mid priority shore power, highest priority generator. You can set the priority however you want, just have to connect each power source to the appropriate priority of the transfer switch. As I said, the transfer switch two input connections are usually labeled "Shore" and "Generator", the "Generator" input has priority. If there is power coming into the "Shore" input and the "Generator" input at the same time, the transfer switch turns on the generator input and shuts off the shore input.

It sounds more confusing than it really is. But if I can figure it out, anybody can.

I wonder is somebody tried to install batteries under truck bed? There should be enough space and that would not only be more convenient, but also lowers COG.
I found this one on ebay designated for Sprinter vans.
For transfer switch, I am making conversion with no generator and I made inverter the default.
When transfer switch is activated, it will draw some power and you don't want it while using batteries.
Still have to remember to turn converter breaker off/on.

Are you using a relay or something to disconnect the converter when the inverter is on? How about the air conditioner? Are you disconnecting that when running the inverter, or do you have a huge inverter and battery bank to run the AC?

Added a small single breaker box and moved the converter 110v supply from the power center to the breaker box. Then wire the breaker box to shore power before the transfer switch so it will only run from shore power.

Didn't take the time to move the A/C ahead of the second transfer switch. Figured if I turn on the A/C while running on the inverter and run my batteries down, I'm a dummy. Best way would be to also move the A/C out of the power center and into it's own breaker box (or with the converter in a small box that will handle more than 1 breaker) ahead of the transfer switch.

I was thinking about adding relay to converter circuit and hook it up to transfer switch, so it would be open when on battery power and close when shore power is on.
But that makes lot of additional wiring and as said above >>> if you forget to flip the breaker when dry-camping you are a dummy.