Relative Work is a term used widely in the UK, and yes, it has been around a lot longer than the term FS...however, 'officially' we now use the term FS. Sounds like you do too. I think RW sounds a bit 1970's, but that's just my humble opinion. I mean, don't even freefliers try to 'work' relative to one another? FS is a more accurate term and, I think, brings the sport more into the 21st century. RW makes me think of white jumpsuits without booties, with pink and blue grips...LOL.
Cheers anyway :p