I was told that the combined capacity of the waste tanks (or the capacity if there is one) must be 115% of the fresh, according to the code.
Code requirements are a matter of state and not federal law, so there is no one single answer here, however, I have never heard of any such requirement. That's not to say that some jurisdiction has not decided to do so. That said, most states have adopted NFPA 1192 as the governing code for RV plumbing systems, and I just combed through my 2002 copy which does not include any such provision. Just to double-check, I looked in the on-line read-only version of the current (2008) code and it's not in there, either. FYI, section 7.5 deals with waste tanks.
I strongly recommend anyone doing a conversion buy the current code, available for $37 from the NFPA. Not only does it cover waste systems including venting and discharge requirements, but also fresh water, LPG, generators, "toy compartments," emergency exits, and other life safety issues. Well worth the money. The code is also available "read only" for free, but you can't print it, copy/paste, or even take a screen shot, as the tool they use to present it locks all that out. You will also have to register on their site with a valid email, which puts you on their marketing email list.
The code can be found here:http://www.nfpa.org/aboutthecodes/AboutTheCodes.asp?DocNum=1192
So I would say, no, you are not required to have 115% of fresh capacity for waste unless the person who told you so is a code official in your jurisdiction.