I agree with using SS strips under the rollers, that is a common solution for the weight issue. Most of the problem is where the weight of the slideout sits on the roller when the slideout is retracted. Going down the road, the slideout will bounce up when you hit a bump and them impact the roller. So make sure you use thick enough SS to handle that impact.
The underlying problem is that the plastic is there to protect the wood from water intrusion from below. But it also will trap any water that leaks in from above and then prevents the wood from drying out. Your fiberglass and resin will have the same problem. So, it has to rot. Weep holes would be better or no plastic, just paint the underside of the plywood with an exterior paint, make sure there are no leaks from above (like around window frames), and inspect frequently.