Ampacity/wire gauge is easy enough, though I just learned tonight from the EC&M reference I'll cite later, that flexible cord ampacity should be derated when ambient temperature is above 86 F.. I hadn't realized that! IMHO that's likely inside the walls of a skoolie in the summer.
Wiring with extension cords is a subject of much debate, sometimes rising to religious fervor here on skoolie.net. This conversation is about the insulation over the wires, not their gauge. I'm also not going to call anybody out, say they're absolutely doing it wrong, etc etc because it's just not a war I'm interested in waging. (I haven't been keeping score as to who has used what, either.) My earlier comment and photo with the damaged wire, as well as the following, are the rationale behind my intention to use conventional home/building wiring methods in my skoolie. I found several interesting references through a search that began with the phrase "portable cord permanent wiring."
Through inspection and handling, the insulation and jacket on normal extension cords is seen relatively soft, pliable.. and easily cut. It seems like a good thing to have out in the open, where physical damage can be easily discovered. It's the old conventional wisdom "don't run a cord under a rug."
Mike Holt is a recognized name in electrical wiring circles. He contributed an article to EC&M Magazine Flexible Cords, Cables and Fixture Wire
which does a nice job of explaining to industrial maintenance people how NEC permits use of flexible cords, aka extension cords. In his words, summarizing NEC, "flexible cords must not be: used as substitutes for the fixed wiring of a structure, ..." The article is fairly short and an interesting read.
For 120-volt systems, the RVIA standard
is NFPA 70, aka National Electrical Code, and it specifically calls on articles 551 and 552.
I think this is a relatively new thing: NFPA allows free access to their standards on their web site. It's a clunky online web viewer designed to making copying the text difficult, but it's not SOOO bad.. This link
might work to take you there. One has to register for a free account and agree to terms & conditions to gain access to the docs.
Article 551 is for RVs. Section 551.47(A) "wiring methods" calls out cables and raceways in accordance with (blah blah blah) in articles in the 300's (note from Mike Holt's article that flexible cords are treated in the 400's). Section 551.47(P)(1) is for "direct wired expandable units" -- slide-out rooms, if I understood correctly. It does call for "flexible cord" in this scenario, but with very specific installation requirements to protect the cord and to limit the spread of fire in case the cord does have a problem. Its separate mention for this specific use implies to me that flexible cord doesn't belong in other places in the RV.
So anyway, with all that said: I might use NM "Romex" cable, but probably I'll be using THHN/THWN, likely of the stranded variety, when I finally get to that point. Probably in hard PVC conduit. I just wish I knew of some decent conduit-friendly plastic junction boxes. The outdoor boxes are $$$, the boxes for the ENT "smurf tube" conduit are too hard to find with any sort of variety.. and using steel boxes with plastic conduit seems like a lousy idea.