Advice for inverter installation

You have a good plan by mounting the inverter as close to the batteries as you can. My inverter is mounted on the front wall of the pass-through area to be close to the batteries. My wiring runs out of the bottom of the trailer which also has an enclosed underside as you described. There is sealant around the the wires where they exit the underside cover.

I also have two transfer switches so the electrical outlets in the trailer are automatically powered when the inverter is turned on. This has been a nice modification.