Bad idea. . If its the floor toxic fumes. If it's the bellypan moisture from the road is going to degradeit. Treated wood is resistant to termites and such. I don't think it would stand up to the abuse from traveling and moisture from the road. Also the weight is , 1/2 inch plywood is a lot heavier than the 35 to 40 gauge aluminum sheetting
would not be my first choice but if you have already done it I think it will be okay. probably adds a little weight. just a little corrosive to fastners might take longer to dry out when it does get wet. lots of popups have plywood floors and no belly pan. If you have not done it yet I would reccomend going with aluminum.