I just wanted to clarify one thing. The wiring would probably end up being used for 24 and 12 volt. Does that make a difference?
Lin, I would advise against using type NM cable (Romex is actually a brand name, for type NM made by Rome Cable) in a low voltage application. The principle reason is that it can too easily be confused with high voltage runs; you would not want the possibility of accidentally connecting the two systems, or of someone cutting into a high voltage cable thinking it was low voltage.
Beyond that, most low voltage applications in a conversion will require crimp terminals or compression connections -- few things will have the proper screw posts for #12 solid wire.
I agree with Brian; get a couple spools of stranded wire for this purpose. I also recommend the DIN color convention of using Brown for ground wires, other colors for hots, and avoid black if at all possible.
We don't use solid Romex for one simple reason. From what I have heard, the solid Romex isn't good in a moving application, because it can get brittle and break.
I hear this all the time. Yet in nearly two decades of working on RVs, I have never, ever seen it happen. Moreover, I have challenged this board (and the other one) to show me even one single case of properly installed and supported solid wire in a conversion suffering a stress failure. (Note that properly installed implies that solid wire can not be used for certain applications; for example, the connection from the generator to the coach junction must be stranded.)
By contrast, I've had no fewer than three stranded wires in my coach fail in less than five years, because the contractor soldered in places where they shouldn't have.
All that said, my own preference is for stranded as well, as I find it easier to install. For low voltage, pretty much anything can be used, with "automotive primary wire" and "bell wire" being two common types. For high voltage, THHN is an approved type that is inexpensive and available at most home stores.
Enclosed wires are good, in fact, you could strip the outer cover and just use the actual wires (black and white and red?) running inside the pipe. ...
I don't have a problem with this for low voltage, but you are not permitted to strip the individual wires out of NM cable and use them separately for high voltage unless the wires themselves are individually marked for that purpose.
Stranded marine wire is a very expensive way to go. A 500 foot roll cost almost 900 at a marine supply.
Marine wire is expensive just because it is marine wire, in the same way that, for example, RV faucets are more expensive than household faucets of similar design, just because they are for the RV market. It's a result of supply and demand, as well as manufacturing quantities, more than a consequence of it being fundamentally "better" or utilizing more expensive materials. In part, you are paying for the listing. Since it is not required, I would not personally choose to pay the premium for the marine listing.
... not acceptable by code,but am not sure why. I am still using it in my coach. It is tinned and will resist corrosion in the worst conditions.
To clarify, you no doubt heard this in regard to high voltage systems. For low voltage, almost anything is acceptable.
Even for high voltage, it is not the case that marine wire is not acceptable by code. What is
the case is that the code says wire used in a coach must be listed to one of several standards that are permitted for that use.
Marine cable certainly can
be listed to those standards. However, listing costs money, and AFAIK, no manufacturer of cable for the marine market has chosen to submit their product to listing agencies for the land-based market. There's no point, since almost no one will pay the marine premium for wire when it is not required.
FWIW, this discussion has come up here many times in the past; just do a search on "boat cable". It is a very contentious subject, so much so that I wrote an article on it for the magazine several years ago.