Here is the full list of all 39 Return to Monkey Island achievements. It takes around 15-20 hours to unlock all of the achievements on Windows.
Reached The End In 2 Hours Or Less
Rescued Wally From Monkey Island
Found Cogg Island
Answered One Hundred Trivia Questions Correctly
Performed The Ole Switcheroo With The Replica Flag
Listed Every Mess On The First Swab’s Report
Braved The Scorched Alaska
Got Hired With All Four Mop Heads
Answered Seventy-Five Trivia Questions Correctly
Attempted To Steal The Cook’s Mop
Returned To The World You Know
Started Part Five
Explored The Entire Ocean Floor
Became A Prize Chum
Gave Stan His Toothbrush
Answered Fifty Trivia Questions Correctly
Found All The Copies of ‘At The End Of The Plank’
Ahoy! More Than Twenty Times
Did Gullet A Favor
Offered To Donate To The Earthquake Relief
Waited Patiently To See Carla
Told Everyone On Mêlée Island That You Are Looking For The Secret
Let The Lumpsucker Escape
Decked Out The Sea Monkey II With Spooky Skulls
Started Part Four
Answered Twenty-Five Trivia Questions Correctly
Got To The Heart Of The Problem
Started Part Three
Collected More Than Twenty Trivia Cards
Died For Real
Swabbed The Hold Twenty Times
Started Part Two
Answered Ten Trivia Questions Correctly
Freed Otis
Thoroughly Examined All Of Wally’s Stock
Convinced Cobb To Tell You About LOOM
Shared Your Luck
Started Part One
Fulfilled Your Restroom Obligations