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Like Texas Granny, we have found egg crate on the OEM mattress to be a wonderful thing! We sleep better on it than on our $1000 mattress at home. Of course, our camping usually involves some strong exercise (usually hiking) during the day, and some fair amount of good wine with a wonderful full supper. Who wouldn't sleep well? So it depends on how sleepy your day makes you, as much as the "quality" of your mattress.
On the other hand, I am still trying to identify the half-thickness innerspring mattress that I saw at the Providence (RI) RV show in the fall of 2001. It wasn't in a TM - I swear it was in a Jayco but nothing on the Jayco web site mentions it. To me, this would be the ideal solution - real mattress, but thin enough so the TM will fold. It may have been a special that the local Jayco dealer flopped down in place of the OEM foam slab - smart dealer! Anyone else seen this?
Texas Granny is exactly right. A "real" mattress like your at-home bed is too thick - the TM won't close.
Bill