If you're legal to instruct, you're legal to instruct in RVs. There is no other hoop to jump through from a regulatory stanpoint. Unless you're talking about a tailwheel model, in which case you would need to hold a tailwheel endorsement.
Yeah, it's that simple.
In reality, insurance requirements will often dictate a certain level of time-in-type, and prudence suggests you be knowledgeable about the model in which you're providing instruction. But if you want to speak from a purely *legal* perspective, there is nothing to do.
I'm surprised the FAA would not know that. Perhaps it's just the person you were talking to?
--Ron