There was no one single method for RMBSs, but S&P's LEVELS model or equivalent was often used with structuring. A common, vanilla structuring was into a "six pack" plus I/O strip, the six pack being AAA, AA, and so forth. In other words, seven tranches. There were many structures, however.
Moody's Mortgage Metrics (M3) was another common model used. Fitch ResiLogic was another.
By way of example, here's an archived prospectus from 2006:
In this case we have five senior tranches (each AAA) and several mezzanine tranches (AA+ down to BBB-).