The fmajor "no-no" is that it requires the suicide cord that lynnmor mentioned, a plug that will have live 120v power at its exposed pins. Related to that, the RV's shore cord also has live 120v power on its pins, potentially shorting out or killing someone.
Actually backfeeding into the RV system may work, i.e. power feeds through the branch circuit to the load canter and then out to the other branches. There are some wiring situations where one or more branch circuits might not be capable of that, but generally it would work. However, you will likely be overloading the branch circuit, taxing it with more amps than it is designed to handle. Mot branch circuits will have a 15A max. IThe branch breaker should trip before any damage is done, but why push your luck?