First, the NEC is a collection of standardized rules. There is no law that says a gov't entity, ie. State, County, Municipality HAS to adhere to these standards. I believe almost all do. Permitting is also under the purveyance of your Gov't entities and I don't think you'll find any that will issue a permit for a moving vehicle. In most locales, a property owner is allowed to work on their residential electrical service without a permit.
LP gas for RV's is regulated by federal DOT and most mfg's will adhere to the RVIA and national Fire Safety standards. Again, this is not a building so no permitting is needed.
There is no procedure for an owner of an RV to work on his rig. You can make it into a spaceship if you so desire. No permitting or restrictions on who can do the work if you have it hired out, no licensing or certification is required.
As RV's become more sophisticated, there is probably a push to have trained technicians that have completed certain course work or are able to obtain an endorsement. Who that endorsing body is, I have no clue.
FWIW, I've worked in building trades since I was a wee lad and am presently a Residential Building Contractor. Because of the nature of what I do, it's necessary for me to keep up on ordinance, code changes, permitting, inspecting, new technologies, and a host of other legal matters. I think folks put too much importance into building inspection, feeling that once inspected, they are immune to any calamity and disaster. The role of a building inspector is to insure that the codes adopted for the agency they represent are adhered to, safe buildings being the result. There is no guarantee that the wooden stair tread in your stairwell won't splinter and your foot go right through it, for example.
If you're not conversant with safety standards, don't have the skills or knowledge, or just don't want to complete projects, the best you can do is hire service and repairs done through a reputable service shop.