As an update to anyone using TFL's bus services in London, the RV1 route listed in Rick's 2020 London Guide (connecting Covent Garden and Tower Bridge via the South Bank) no longer exists. While preparing for a recent trip, I wasn't able to find any route information on TFL's website, or on other travel sites. After a bit of research, I found a few reports that the RV1 route was deemed redundant and withdrawn in June 2019 (https://www.london-se1.co.uk/news/view/9898, https://www.southwarknews.co.uk/news/rv1-tfl-service-axed/, https://www.mylondon.news/news/zone-1-news/tfl-bus-routes-scrapped-16134311).
Unfortunately, there isn't an identical replacement route, but both the 1 and 15 busses provide approximate facsimiles. The 1 follows a similar route to the former-RV1, along the South Bank, but doesn't actually stop at either Covent Garden or Tower Bridge. The closest bus stops are Aldwich (~5 min walk from Covent Garden) and Griggs Place (~10 min walk from Tower Bridge). The 15 takes a northerly route along The Strand/Fleet St/Cannon St. As with the 1, the closest stop to Covent Garden is Aldwych, but there is a convenient Tower of London stop adjacent to Tower Bridge.
I've submitted this feedback to the RS guidebooks team, so it should be corrected in future versions, but hopefully this post is helpful in the near term.