A:
This is made for computer use.
If I read your question right:
Two twelve volt, 6ah batteries in series operating a 24 volt system,
No you cannot use additional outside batteries,
Figure about 6 minutes if you use it like you stated before the internal batteries explode into a fireball (which may void the warranty)
They make systems for motor home power to operate from deep-cycle batteries, sold at places that sell motor home stuff.