RPG III and RPG/400 are many years obsolete, although there are probably lots of those programs still running in applications. (We still have quite a lot.) But the prospects of just being a maintenance programmer for that old code are doubtful, and not that desirable either. Also, more and more of these programs are getting converted and redesigned, or getting replaced with newer approaches.
And for new development, as a minimum today, you need to be using RPG IV in free format, and understand ILE concepts and be able to apply and implement them. Knowing other languages and architectures like SQL, Java, PHP, and even .Net will get you much further as well. There are so many ways to go, so it would also depend on the strategy a potential employer has chosen.
As far as RPG prospects in general, it depends on where in the country you are looking. But overall, the market for RPG programmers is not as strong as it once was, unfortunately. Whether that will change in the future is hard to say, but it has been on the decline in recent years. Some think the demand will increase in the years to come due to the amount of code still in use, as more RPG programmers retire, while fewer are now learning that skill set to replace them. On the other hand, with fewer people having this knowledge, more and more companies will eventually abandon these applications as their costs to support them go up due to dwindling resources.