If the particle board is finished on the outside and you don't want to lose that, then perhaps line the inside with some 3/8 plywood. Use construction adhesive. Install some 1" x 4" 12 inch long boards to the underside of the mattress plywood. Install a couple struts between the new plywood and boards.
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2019 Outdoors RV 24RKS Titanium
1200 Watt Solar - 420 AH LiFePO4
2020 GMC Sierra 3500 AT4 Duramax
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