Comfort Without the Bulk
When I was looking to redo the V-Berth on my boat, I researched mattress options, and ran across several 'under-mattress' systems and ended up going with the Froli Modular SleepSystems.
The big challenge is ventilation under the mattress.
Affordable and well made
Easy to follow instructions
Easy to assemble and use
Comfortable and adjustable
As others have noted, it can be a wrestling match with the Froli to get to the lockers under the cushions in the V-Berth. What has worked for me so far is keeping the overall pieces small enough to handle, instead of creating one large connected piece, I have three smaller pieces: One on port, one for the bench starboard locker cushion, and one for the center key piece.
It's not like sleeping on a king sized Casper mattress, but it's comfortable. I was able to keep the V-Berth cushions to 3" and still have a relaxing berth and a good night sleep.