Electrical Additions

[GypsyFamilyRV]

Hi! I'm making some additions to my rig (2007 Keystone Montana Mountaineer), and would like some input (from someone with more electrical knowledge than me!).

Basically, I'm adding a battery box/bank with 4-6V batteries, with a selector switch, and keeping the original battery as a backup. In addition, I'm adding a 3000W power inverter, and a portable solar panel.

Here's a few notes:
--I've hardwired the solar connector onto my batteries as shown in the diagram, but only plug it into the panel when parked, so its not hot all the time.
--To keep it simpler for me right now, rather than wiring the inverter into the coach's fuse box panel, I plan on running my 50A cable from the coach, with appropriate 50-30 and 30-20 amp downsizers, and plugging that into the inverter. I realize I can't run my AC on that, and that's fine, I wouldn't have the battery power for that anyway.
--Provided my assumptions and the diagram are accurate, I believe the last thing that I need to do is to wire a kill switch onto my converter, so as not to create a power loop that drains the batteries.

So basically, I'm looking for advice on 3 things...does the diagram look sound, or are there any recommendations? Secondly, where/how should I wire a kill switch for the converter? Killing the breaker switch for it is not an option, as the converter is on the same breaker as the power outlets in the rig. Is it as simple as wiring a switch into the point on the drawing with the 3 question marks, and turning it off there? And lastly, should I be able to just charge the backup battery by setting the marine selector switch to setting 2 while the converter is on?

Any help is greatly appreciated! Thank you!

Sean