RTW is a good call.
It's called an 'open jaw' fare. I would say that travel agents can't access any different fares from the better travel comparison websites, but they can usually find quirky rates much more easily.
There are numerous websites that 'blog' cheap open jaw and return sales (an expired example), and although it is a pain to keep up with it, it is probably worthwhile waiting for a sale event from the major airlines before you book anything (unless you are travelling within the next few months).
I agree with the comments to look at other variations in the flights. For example, if you split the journey up at KL or Singapore you might find ticketing separate return/open-jaw journeys with a break cheaper than the open-jaw return. Similarly, flying from or to a European city and getting a cheap Easyjet type flight there. Assuming you take the stopover as part of the holiday.