Has anyone else had this problem? I completed RTK about two weeks ago. I've been sort of keeping up with reviews (every other day, I clear out the expired stack in one massive review). What I've noticed is some of the kanji in the fourth stack are slipping. Kanji that were very familiar to me and I passed without trouble now draw complete blanks when I see the keyword. Is this normal? Being in Japan, I see the kanji every day and I'm working on learning their readings and their use in compounds, so is it just that my brain is substituting Japanese stuff for the English keywords? I can often recall the keyword when I see the kanji, but have trouble the other way around, even though I only review from keyword to kanji.

I heard that the review system on this site spaces cards a little too far. Should I start using Anki for review instead?

It's completely normal. Even though you "finish" RTK, ie: have gone through the whole book, your brain is still consolidating what you have learned, connecting it to japanese readings, compounds, etc., as you're experiencing. There's a lot going on up there and sometimes things slip through the cracks, so to speak. It's frustrating to forget kanji you thought you had nailed down, but it happens. Throw it back in and continue.
As for whether to use this site or anki, give different systems a try and see what works for you. I actually use both. If you check out the maximum period for review in anki, it's set by default to five years, I believe, which is much longer than the review period on this site, though you can change the preferences.
Anyway, the more sources you have throwing kanji at you: This site, anki, books, signs, etc, the faster you brain becomes really familiar with the kanji and you get to the point where recalling is automatic.