I think most RV electrical panels are accessed by pulling them from the front. I know for sure mine is. (I'm not talking about just removing the cover over the circuit breaker wiring, which always is from the front, but about getting at the wires etc. behind the panel.)
At any rate, if you wish to power the entire AC electrical system from the inverter, using an additional transfer switch, you can put the transfer switch in a few different places, and I think where it ought to go depends mostly on what priorities you would like for the various power sources. I'll attempt to illustrate with rather crudely drawn diagrams.
With just a generator and a shore power connection, the generator typically has priority over the shore power (and a delay before the transfer switch changes over to permit it to get up to speed and under control), like this. I'm using a start to mark the priority input to the ATS.
If you want the inverter to have priority over both the generator and the shore power connection, you can connect the new transfer switch after the existing transfer switch with the inverter having the priority input, like this.
If you want the inverter to have priority between the generator and the shore power input, you can connect the new transfer switch between the shore power input and the existing transfer switch, like this, giving the inverter the priority over the shore power.
If you want the inverter to have the lowest priority, you can connect it similarly to the previous except swap which input to the new transfer switch has priority.
If your shore power cord is permanently attached to the RV and gets stored in some sort of a compartment, I would suggest not getting a transfer switch and instead simply installing a socket in the storage compartment to plug the shore power cord into. That would cost considerably less money, probably be easier to wire, and is not a significantly greater amount of work to use in typical cases as you simply plug the cord in when you stow it.
My personal opinion is that powering the entire RV 120V electrical system from the inverter is rarely the best option. I would rather have selected circuits that are either switched or dedicated to inverter usage. That's personal preference, not absolute gospel truth.
