finishing installing Battery Disconnect and power inverter

Inverters mfg today
Most will draw something, when off
Because they are not truly off

The switches are control switches, not main power switch, they are too small to carry even the 25 amp load of a 300w inverter
No way are they going to carry the power of a larger inverter
If they built them with mechanical electrical relays operated by the switch
There would be no drain when off
But it's all electronic controls, and there is always some drain,
For storage mode you need a mains power disconnect

My Xantrex 1800 seemed to draw power even when switched off. Not much as I would guess just a few milliamps. Never measured it but it was enough prevent my 15 watt solar panel from keeping the battery charged. I would remove the main fuse in storage.

An inverter on and idling will draw almost 1% of rated power.

With mine, as long as the inverter power switch is off, no power is drawn.

If the inverter has a switch that eliminates any 12V draw, you can install a battery disconnect anywhere you want. Typical inverters behave like that and there are some that automatically do that but are in standby until 120V power is needed.

For your application where assumed storage disconnect is needed, install the disconnect close to the battery. If you choose a master disconnect switch with the right terminals, you can move the CO2 alarm and the slide to the load side of that switch. The only circuit that needs connection to the battery beyond a disconnect switch is the emergency break-away system for braking. Leave that directly connected to the positive polarity terminal of the battery. The reasoning is that even if you hook up and drive away with a battery disconnect switch in the "off" position, your emergency braking system will still operate.