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