RTL is implicit in your FAA licence
I was in the same position as you. I did do the CAA RT licence (had huge problems with the practical and got ticked off for using the word Point rather than Decimal). The Theory was easy.
I later found out that you can just ask the FAA for a RTL. They don't bother Stateside but for flying overseas u might need to produce it.
In practice doing the CAA one is still a gud idea tho'
Good Luck
mmd