gkreutzer - I have thought about adding a second battery myself. If you read all the posts in this thread, you will find someone who flattened the lip and installed a second battery turned the other way. Believe it was Gary Swanson who placed the bottom of a milk crate in the bracket and was able to install two batteries in a raised position.
On the inverter location, you would want it close to the batteries; but you would want a cut-off switch accessible from the inside of the unit.
Gene
2002 Trail-Lite 211S w/5.7 Chevy