If you plan on leaving rubber in, I would think a layer of thin luan, (waterproof plywood), would be a good "base" for your laminate floor.
I might do that to mine in the interest of saving time. I'm probably not going to be around or driving in 10 years anyhow, so no real concern about rust under rubber.
My bus is from NC where no salt was used on roads, so there shouldn't be any real rust issues under the rubber.
After a thorough visual "upskirt" inspection, the floor is totally solid underneath with no rust on any structural members or floor underside.