Error in .local(drv, ...) :
  Failed to connect to database: Error: Access denied for user 'user'@'localhost' (using password: YES)

library(RMySQL)

mySqlCreds <- list(dbhostname = "dbname.rds.amazonaws.com" , dbname="dbnamefull"                 , username = "user",pass = "secret", port = 3306)

drv <- dbDriver("MySQL")

dbConnect(drv, host=mySqlCreds$dbhostname, dbname=mySqlCreds$dbname, user=mySqlCreds$username, password=mySqlCreds$pass, port = mySqlCreds$port)