Trekwood.com is a internet parts store associated with Tiara RV Sales, a Keystone dealership. The Trekwood website has listings for all the mattress configurations for the Dutchman line. Essentially, the "extension end tents" had 3" thick mattress pads (not mattresses, but mattress pads) while the fold up sofa had a 5" or 6" thick mattress.
You can look at what's available at this link:
https://www.trekwood.com/parts-searc...arch-keywords=
I would recommend using that website as a reference, but not as a source for buying replacement parts that require shipping. Trekwood's shipping charges are, for many items, way more expensive then the parts you're buying...
Trying to replace a 3" thick foam pad with a quality mattress may not give you the versatility needed to store the end wing tents for travel without having to pull the mattresses from their location and storing them elsewhere in the trailer during towing.