The IBM RS/6000 has a POWER architecture CPU, possibly a PowerPC 603 or PowerPC 604, or possibly one of the newer models like POWER1, POWER2, POWER3, etc. The most recent (current) systems use POWER7 or POWER8.
Anyway, if the system has the compiler and toolchain installed on it then there should be a decent symbolic debugger included, and you should be able to use that to disassemble any executable. Depending on exactly which version of the OS it was compiled on, and which compiler was used, you might even be able to use PowerPC tools on some other OS, such as MacOS, or even potentially a cross-compiler toolchain on any type of system, to disassemble the program. For example GDB built for PowerPC may be able to disassemble the program.
However if the executable has been stripped of symbols (as was typically the case on AIX systems, IIRC), and especially if it had been run through the most powerful optimizing stage of the compiler, then you'll be pretty much lost and what you are trying to do will be impractical and require many man hours to decipher -- indeed many thousands of man hours for any significantly sized program, even if you're able to hire someone to help who is familiar with the code generation patterns of the particular compiler which was used.
You might be better off trying to hire an archeologist to help you dig through the specific landfill where you might hope to find listings or backup tapes or CDs or disks containing the original source code, or specification documents, etc., for this program. Seriously.
Or try to find and (re-)hire the original author(s).