I had a Keystone OUTBACK that had a couple spongy spots on the floor. To be fair, it was a pretty light trailer...and they build them 'light'.
It was never a problem, but you could feel the give. But as with most RV trailers....don't expect any of them to feel like your floors and walls at home. You wouldn't be able to move them.
The SALEM trailers come with 5/8", 5 ply tongue and groove plywood decking, and 3/8" plywood roofs.
Pete (Jolly Mon)
2014 Jayco Whitehawk 24 RBS
2004 Nissan Titan LE, 4x4