We bought an 8 inch memory foam mattress from rvmattressbargains.com to replace the one in our Solera 2011 24S. It is rectangular w/o the corner being rounded and that's no problem getting by. Cost was slightly over $600. The mattress is extremely comfortable and there are no sinkholes...and I weigh over 225.
We were at the Tampa RV show yesterday and saw new Soleras (24S and 24R). They have wonderful foam mattresses. Firm, comfortable, no hole in the middle. Solera owners should be calling FR to see if you can get one. Now if I can figure out what to do for my Sunseeker 2300.
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2012 Forest River Sunseeker 2300 Chevy