My house battery cables pass from the battery bay, through the back wall, into the inverter bay. There's a grommet there to protect the cables passing through that wall. I'd like to get a few more for a refrigerator inverter install that will require a couple of new holes through the bay walls. Does anyone know the size, or a part number for that grommet? I'm sure ebay or Amazon would have something if I can identify it.
Try Ace Hardware they usually have a selection of grommets
Something like this perhaps, 1-1/2" hole, insert one from each side,
Amazon.com: The Hillman Group 59339 1-1/2-Inch Grey Grommet without Cap,... (https://amazon.com/Hillman-Group-59339-Grommet-without/dp/B00NTW2G0I/ref=sr_1_9?ie=UTF8&qid=1473803369&sr=8-9&keywords=1-1%2F2%22+wire+grommets)
Wire penetrations from your battey bay to the inverter bay should be sealed. A 3M 4000 sealant will seal and provide protection at the same time.
The Amazon part that you suggested is sort of what I had in mind. It's very similar to what is currently there but I'm not sure that the existing hole is 1-1/2" so I'd need to measure.
I don't disagree that sealing the battery bay is a good idea but that doesn't seem to be how mine was built. All of the cables pass through the wall, with only the grommet to protect them from the aluminum panels on both sides of the hole. It's almost completely sealed just from being filled by the wires themselves. They're packed pretty tightly. As far as I know, this is original factory work.
I also have another hole that's the same size between the battery bay and the adjacent joey bed that has been filled with a plug. My plan was to remove that plug and route #2 awg battery cables, with a 150 amp fuse on the battery bank side, to a new 1200 watt inverter that will then power the fridge. It's this 2nd hole that needs either the grommet or sealant. Considering the size of the hole, I think I'd be better off to drill a new, smaller hole if I use sealant.
These grommets come in many sizes, get one that fits the wires with some room to spare. Use a hole saw to drill,the hole. Get a good magnet to locate internal steel framing in the bulkhead.
Generally the battery space should be vented (usually through the floor) and the isolater should not be in the same space and the spaces should be isolated from each other, ie sealed. This is especcially importand with flooded batteries. Sealed batteries, not so much. Using 3M 4000 will seal and protect the battery cables.