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