I used fiberglass rods to fish the wire through to the slide where the fridge is located. Just follow the current wires. I did not have to open up the belly. I guess I lucked out for a change. See the photos.
Below is the battery compartment. On the back wall, there are holes there where the wires come through. I ran my fiberglass rods through those openings and ended up behind a plastic cover. See another photo.
Attachment 12671
Below is where the fiberglass rod came to. That black wall that the yellow line is on is plastic and held in place by a screw or two. Remove the screws and the rod was sitting right there. Also note that the plastic cover has a notched corner for the wires to come through.
Attachment 12672
Below. Once wire is through, just follow the other wires and zip tie it to the other wires.
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Below I drilled a 1/2 hole to get the wire into the slideout and behind the fridge. Before drilling the hole, make sure you will have room enough to clear any LP or electric lines and to make sure the back of the fridge will not hit the new wire.
Attachment 12674
Below I bought a 10ga inline fuse. It came with a 40amp fuse. Do not use the 40amp. Replace with a 25amp and
I tied it into the that red terminal block which comes directly off the battery.
Attachment 12675
Your RV will be different I would think. Mine is a 2014 3402RL. But thats how you should be able to get the wire through. Hopes this helps.