Our 2012 Lexington came with an"upgraded" (yukyukyuk) Serta Pillow Top.
Lasted about two years, and we suffered with it for another year after that.
Checked around for gel top memory foam mattress'es. Found a great company on Amazon...Dynasty Mattress.
Bought a 10 inch Gel Memory Foam, RV Queen from them (price is $289.00), with free shipping for Prime Members. Thats almost as cheap as buying a quality gel topper and using it over your old junk mattress.
Best $289 we ever spent. Firm, but not hard, and after 2 years of use, it's still as supportive as when it was new. Our last trip this year was 13 weeks long, and no problem sleeping on it for that length of time.
A bit heavier than a standard spring mattress, but after two years the bedroom slide (where bed is located) works just fine. Maybe 20 lbs heavier.
Actually thinking of getting their King mattress for our home.
Steve & Cheryl + Zoey, and Ziggy, our furry kids.
2012 Forrest River Lexington 283ts
Toad, 2013 Subaru XV Crosstrek