1st salvo: 3" memory foam topper (trimmed to fit) works for a while.
You can have a foldable memory foam mattress made for your rig, to your specifications. There are several vendors who will do that for you; one example:
https://www.googleadservices.com/pag...5wQ0Qx6BAgMEAM
We replaced ours with a Zinus from Amazon, but we just needed a short queen, no need to fold it in our rig...
But if you're handy, you could purchase a foam mattress, cut it down, and fashion some kind of way to fold it...
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-2018 Greyhawk 29MV
-2020 Jeep Wrangler Unlimited (JLU) (Primary Toad)
-1994 Jeep Wrangler YJ (Secondary Toad)
-2014 Jay Flight 28BHBE & Ram 2500 6.4L CC 4x4 (sold)