Is your "large amount of RF" being INDUCTIVELY coupled into the power wiring? If so, it is indeed very strong and using shielded cable won't help. But if it is CAPACITIVELY coupled, then shielding MAY help depending on other factors not in evidence here. Perhaps we should also ask why you don't simply consider using proper entrance filtering at the load end of the power wiring? That way you wouldn't be dependent on using a particular kind of wiring. Without knowing the full picture (and what "large amount" means), it can only be discussed in very general terms.