Inside the much-publicized 50th anniversary concert tour of the Rolling Stones, which is spurring a frenzied quest for information and access to the so-far limited dates, there’s another band milestone this year — one that’s perhaps essential to the vitality and durability of “The World’s Greatest Rock and Roll Band.”Legendary keyboardist Chuck Leavell just quietly clocked his 30th year on the road with the band, serving not only as the man on the bluesy, rocking ivories, but also as unofficial “musical director,” sorting out every evening’s show with frontman Mick Jagger. “There is such a wealth of material that it is, frankly, a constant challenge to decide which 20 or so out of that deep, deep well will make it to the stage on any given night,” Leavell says. “And don’t forget that we need to insert the new songs in there.” Leavell says there have been several songs during his 30 years with the Stones that the band initially thought wouldn’t work well, but wound up being worthy of a spot on the setlist. “For instance, ‘Can’t You Hear Me Knocking’ posed a challenge for the band when we first decided to try it,” he explains. “It’s not exactly compact. But I pushed for it, (because) I felt the fans would like to hear the band stretch out on something. It has a section that is sort of a jam, with a long sax solo, harmonica solo and guitar solo. “It’s not the kind of song that we necessarily want to do every night, but when we dust it off and perform it, we always gets a great reaction.” Given the famously volatile relationship between guitarist Keith Richards and Jagger, Leavell’s success is no doubt aided by what the venerable musician calls a “think before you speak” rule that he says applies to “musicians and the media.” Just before leaving to begin band rehearsals in London, Leavell says the band is in top form, noting that lead guitarist Ron Wood, the subject of tabloid headlines due to numerous rehab stints, is now “in the best shape since I’ve known him.” Fans of Southern rock know that Leavell’s place in the rock pantheon doesn’t depend upon his association with the Stones; Leavell spent time in the early ’70s in the Allman Brothers Band, fronted his own outfit, Sea Level, and has played on hits with Eric Clapton, George Harrison, the Black Crowes and most recently John Mayer, among others. While Mayer’s hugely anticipated summer tour was cancelled due to his recurring throat problems, that didn’t stop him from bringing the band together in Montana this summer to continue recording and making a video. Mayer describes Leavell as “a Southern gentleman through and through” and someone who’s confident and clear-headed. But he also waxes rhapsodic about terms only musicians can make sense of, admiring Leavell’s “Hammond B3 drawbar settings” and crediting him with adding the keyboard flourishes that nicely fill out such Mayer gems as “Queen of California” and “Age of Worry.” Seemingly calm in the face of the storm that will be another Stones mega-tour, Leavell’s outward concern is for the environment; he’s been honored many times over the past three decades for his work preserving and protecting forests and farmlands in the South, and also for co-founding the Mother Nature Network MNN. Which may provide another clue to the inner workings of the Stones. If you listen closely to the just-released “Doom and Gloom” album, the first new Stones record in seven years, there’s a line that talks about “Fracking deep for oil, but there’s nothing in the sump.” After 30 years with the Stones, it’s quite possible the “Southern Gentleman’s” influence goes beyond where “Memory Motel” fits into the setlist.