Kind of reading between the lines, you're not suggesting plugging a dogbone adapter into a 30 amp socket and then plugging in an extension cord with a 15 amp plug into the circuit to power your RV, Right? If you are going to do that you should use a 30 amp extension cord.
So if your not plugging that cord into your RV, what are you going to plug into that 15 cord and why would you? Doing so would override the safety factors built into the NEC and any possible fire caused by over heating the cord your responsibility. It just doesn't seem logical or necessary and you seem to be fully aware you are doing something you should not do.