The under bed plywood from the factory is 3/8ths or less luan. The stringers seem to be spaced on 16-18 inch centers. Be sure to have full edge support on both long edges and some support on the ends. In My experience having an extra slat in the middle of the mattress (where you would sit down prior to stretching out) adds some nice strength. Attach the plywood to the edges and let the wood float on the stringers. This reduces the chance of snapping a stringer from flex of the ply or weaken it with holes.
Edge screws are normally every 12 inches. Look in the flat steel bin at Lowe's or HD they have 1/2 inch up to 2 inch L shaped Aluminum that can be cut to brackets or works to run on the wall as a edge for the inside of the framing. The factory uses a box aluminum channel that may be available locally from a screen room co, eliminating the wood and the weight.
Remember, you don't need to design it to hold 100% of your weight in one spot. The mattress will spread it over the frame.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
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