I guess the real question is has it been presented to the UL for listing and failed, or has it never been presented for UL listing for use in RVs? My guess is nobody has asked the UL to list it for use in RVs since no RV manufacturer would go to the expense of using it.
That's not quite how it works. UL is a for-profit testing laboratory (and not unique; there are other labs, and the code recognizes many of them as legitimate listing sources). Products are not tested as a category, IOTW, just because Rome Cable submits a sample of its Romex brand NM for testing, and UL finds it acceptable and assigns a listing number, does not mean that, say, Southwire can just market its brand of NM cable as being UL listed. Even if Southwire's product is virtually identical to Rome's, it still must submit its own sample to a lab, the lab must do the mandatory tests to all applicable standards, and, if the wire meets the standards, it will be listed and be assigned its own, separate, unique listing number.
So in order for boat cable made by, say, Ancor, to be listed as Type NM (which would make it acceptable under the NEC), Ancor would need to send it to UL (or whomever) and pay, if you will pardon the pun, a boatload of money to have it certified to that standard.
Now, note that Ancor does, in fact, already pay UL a boatload of money to have the wire tested and certified to an entirely different standard, which is the one required for the marine industry. That expense makes sense, because without that listing, they'll never sell any wire. However, it makes no sense to spend the money on listing to the NM standard because no one will buy boat cable to wire a house, or a business -- you just can't justify the extra expense. Even the commercial RV industry will generally not pay the premium for boat cable, because existing NM, at roughly 1/5 the price, works fine and is approved under the code. So that leaves a handful of DIY busnuts who might be interested in buying this product if it had the proper listing -- not enough to justify the hundreds of thousands of dollars UL charges for the testing.
I wrote an entire article for Bus Conversions on this topic a few years ago, which I would be happy to repost here if anyone would like to see it.
Is there a listing of what the UL listing number means the product has been approved for.
UL listing numbers, as I wrote above, are unique to each item tested. You can get the manufacturer, product, and standard for every UL number from UL. However, wire is also required to be marked with its type approval, which is probably more useful information. Boat cable, for example, is listed type "BDFX" and if you look that up, you will find the relevant standards and approved uses. "Romex" and other brands of similar cables are listed Type NM, which is a specific standard within the NEC. There are literally hundreds of listed "types" of wire; your shore cable is probably type SO or SJ (hard service or junior hard service cord), large battery cables are often type DLO (diesel locomotive cable), and most commercial wiring for branch circuits is done with type THHN, THWN, or THNN. Note that many wires carry multiple type listings; it is not uncommon to see THHN and THNN together. It is entirely permissible for a single cable to be listed Type NM and Type BDFX at the same time, but, to my knowledge, no manufacturer submits any wire to be listed to both of these types.
You might find this UL guide to wire and cable markings useful:http://www.ul.com/global/eng/documents/offerings/perspectives/regulators/electrical/newsletters/W&CMG_April2007_Final.pdf
Sean, I used NM cable in 10 AWG to wire a circuit in my house. Maybe I read your posting wrong, but you seem to be saying that it should have been stranded. It most certainly was solid copper.
Well, #10 is actually a dividing line of sorts. Typically, NM up to #12 is solid, and many manufacturers start stranding at #10. AFAIK, there is no solid #8 NM, it's always stranded. Mind you, this is very coarse stranding, compared to the rather fine stranding of boat cable. If you need the flexibility, either for installation reasons or due to flexing under way, then something like THHN with finer stranding is called for.
... That said, you can buy type NM in 8-2 or 8-3 stranded at any electrical house and it will cost quite a bit less than boat wire. You might even find 8-4 with a single ground that I think would serve your purposes with a single run. I don't think there would be any prohibition against running the in and out of the inverter in the same cable. Sean?
Len, this is an excellent question, and I will have to research the code and get back to you. Off the cuff, though, I will say it is not acceptable, if for no other reason than color coding. The code requires the neutral for each circuit to run with the hot (so a neutral needs to run each way, to and from the inverter) and it also requires the neutral to be white (hot wires can be any color except white or green). Since 8-4/wg would only have a single white wire, there's no way to meet the neutral color requirement. In very large gauges (where wire is typically only made in black), the neutral and even the ground is permitted to be marked with colored tape at the ends, but this is not a legal option in the smaller gauges.
Your point that 8-3 NM is certain to be stranded and will be less expensive even than 10-3 boat cable is a good one.
BTW, FWIW, every wire that has failed in my coach has been a stranded wire that was, against my recommendation, soldered into the crimp, and the wires later broke off at the joint. None of the solid wire in my coach has ever had any problem.
(Edited to add link to UL Wire and Cable Marking Guide)