As for learning 音読み, RTK II's usefulness seems to be somewhat questionable, but what about vocabulary? This thread would seem to indicate a good deal of the compounds provided just aren't that common and thus it wouldn't be terribly useful for learning functional vocab words.

I was interested in getting some ステップ books for this exact purpose, but that would be a wee bit difficult currently, whereas I already have access to a copy of RTK II: I'm just unsure if I should actually be attempting to use it. This similarly applies to the latter half of RTK III: I plan on completing the traditional (well, traditional from an RTK I perspective) half of the book, but what of the reading section?

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If you have access to a copy you could do just chapters "Readings from Every Day Words" and "Readings from Useful Compounds", that will give you about 460 compounds.

I don't know if RTK II is the best vocab builder, but skimming through I recognized the majority of the compound words in there, so it didn't seem like that many were obscure.. Some kanji that are used primarily for their kun yomi will of course have more obscure onyomi jukugo, and some of the RTK I kanji just aren't that common so they too will have compounds that aren't necessarily that common. But since you have it at hand you may want to use it.

"Kanji in Context" gives a good amount of vocab for all of the jouyou kanji plus two additional kanji. They say they include about 8,ooo words. So this would be a more comprehensive option that also gives really good English translations for the words. And if you get their two workbooks, you will also have a source for sentences.

I also like the "Basic Kanji Book" and "Intermediate Kanji Book" series that has two volumes at each level. Unfortunately they only cover their pick of the most important 1,200 kanji. (I wish they would put out "Advanced" and at least finish off the jouyou.) But is starts out easy and builds to at least a high-intermediate level. And instead of isolated sentences they offer reading passages from newspapers, etc. They also help you build the skills for recognizing how compounds are constructed and other things.

As for RTK III, I have been frustrated by some of his obscure compound word choices that reflect the keyword he attached rather than a different meaning in a compound listed as common by edict. But I haven't gone through enough of RTK III to judge how common this is yet.