I have also installed my INVERTER in my passthrough storage as it is the closest convenient place to the batteries. I went with making a hole through the floor in there, then out through an existing 1" diameter hole in the frame to the batteries. I've got a set of 2-gauge wires (positive and negative), plus a couple other small diameter wires needed for my stuff (battery temp sensor, battery voltage sense wires, etc.). The wire run length is about 5' from battery posts to the inverter. I installed my inverter and solar CC, along with circuit breakers and battery disconnect switch, to a panel that I mounted to the interior of the front wall in the pass through. There is some worry about items moving around in the pass through making contact with stuff, but in 10+ years I have not had a problem. KNock on wood. My setup has the wires coming out beneath the pass through floor into the space between the coroplast and camper floor, then out through the existing hole in front of the frame, then to the batteries. I used wire loom to protect the exposed wire between frame and batteries.
The plastic sheeting is called coroplast and a local RV dealer should stock the stuff in a roll that covers typical RV frame widths. Or Home Depot will have it in smaller stock sizes. If you have to cut through yours to pass wires through, seal it up good with foam gap filler or something. Otherwise, wind can work its way in there at highway speeds and it will puff up and tear away from the camper. I experienced this and lost the entire coroplast sheeting. That's how I learned that the RV dealer stocks it in a roll for purchase.