I agree that you shouldn't use Romex (NM or NMD) in RVs. It is rated for 600V and temperature range of -10C to 90C but solid conductors and a stiff PVC jacket transfer any vibration to the terminations. Additionally, with the large temperature swings that an RV sees compared to internal residential wiring, thermal expansion of the conductors could further loosen standard wire nuts (which are also not suitable for vibration). If you do have Romex wiring, crimp on or set screw splices / wire nuts should be used at a minimum.
There are better options:
Service cable (AKA Cab Tire) - SOOW, SJOOW, SEOOW and SJEOOW. Basically rubber (or elastomer in the case of the E cables) coated, flexible, water/weather resistant, oil resistant, stranded electrical
cord. The SOOW and SEOOW are thick, heavy duty and 600V rated while the J (junior) variants are thinner and 300V rated. The E variant has better chemical resistance and is rated to -50C vs -40C.
Marine Triplex (Ancor Triplex) - is also stranded and weather, chemical and vibration resistant, but it has tinned (solder coated) wires to resist marine corrosion and a stiffer PVC jacket (rated to -20C to- 105C). Generally RVs don't experience the corrosive sea water environment that boats do. It has a less flexible PVC jacket and comes in round or flat profiles. It is similar in price to SJOOW.
I would use SJOOW or marine Triplex for internal circuits and SOOW for your shore power cord.