A good source of by-the-foot wiring is Genuine Dealz It's also marine grade cabling. I don't like it quite as much as Ancor (only because the insulation is slightly thicker and harder to work with) but it does make the official grade, and have no problem at all using it. I do buy it when, for example, I need smaller amounts of multiple colors. Another good service they have is custom battery/large cables.
To make proper crimps takes some expensive tools. I bought those, but I make a number of crimps, plus I'm a sucker for buying the tool vs. paying someone else (also I had a poor experience some years ago where "pro" crimps failed). That said, this summer I helped a buddy re-wire the core of his brand new trailer (because they still don't really set them up all that well) and we were away from my big tools and wire supply. I decided to try the Genuine Dealz custom cables option. I was actually quite pleased. First of all, the web page for "building" the cables is very good. It's easy to see what you are doing and you can customize the wire color, shrink color, length, size, etc.. Even better, the cables seemed quite well made. Granted, I didn't test them to destruction (because my buddy needed to use them), but nothing jumped out at me as being horrible.
One disadvantage of buying the pre-made cables - especially if they are shortish cables of large wire - is that you can't specify how the lugs are oriented. Sometimes that can be critical in small spaces, and you can do them however you want when making your own. But I would rather have their decently made cables than home-made ones with a crappy crimp.
Genuine Dealz also has great, fast shipping. They also carry my favorite FTZ power lugs (for large cables).
I find that most RV's aren't wired up to a very high electrical standard. By that I mean that cables are overly long and thin (voltage drop), may not be properly fused, may be fused but with too low an AIC capacity (so fuse holder/fuse blow apart independently of fuse size), and may not be well secured (cushion clamps) or protected (anti-chafe). That kinda sucks, but on the other hand it can be very satisfying to clean it up. And hey, there is very little sawdust, fumes, or mess
(On the other hand, RV's aren't burning down right and left, so they do get by.)
