Odds are, the mattress itself distributes your load to where the plywood doesn't deflect as much, and you aren't sleeping in the sitting position, so your weight is spread out over a much greater area, again reducing the amount of sag in the plywood.
My money is just on a crappy mattress. Mine had a depression in the mattress where I laid at night with just 2 weeks of usage. I'm in the process of getting a cash out warranty claim from Jayco as we were on vacation and I had to replace it on the fly.
2017 Eagle HT 26.5 BHS 5th Wheel
2014 F-350 Lariat 6.7 CC SB SW
B&W Turnover Ball