I just replaced mine. If it is the spring mattress with the inflated mattress inside it, I bought it from Airdream. The thickness measurement is with the air mattress inflated and the spring mattress included.
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2007 Fleetwood Excursion 40E Cat C7 350 HP
2007 Chev 2500HD D/A Long bed CC (Yes, it is my TOAD :B)
2011 Toyota Tundra
2013 Jeep Wrangler Unlimited Sahara (Backup towed)
Gone but not forgotten, 2008 Jayco 299 RLS