There's alot of folks, me included, that are using extension cords. I calculated that 10/3 would be nice to have in my toy hauler with the AC and 45 amp panel. When I looked at 10/3 romex, well, I wondered what else I might try

I picked up a 100' 10/3 extension cord with a triple end from eBay for $60 delivered. I only used 60' of it, so now I have a 40' 10 gauge extension cord too
The blue wire in the image is the extension cord. The sheath around it is very robust and much more difficult to penetrate than Romex sheathing.
If you're doing a smaller build, any old extension cord will do. I had an old 12 gauge laying around with both ends cut off. It had an abrasion in the middle, so that's how a jobsite safety guy will "disable" your cord for you at the Space Center. I've used it all over the hauler for wiring on outlets and such.
Good luck
