I reckon all Jayco vans would suffer the same problem.:gossip:
It looks as though the ply on the floor is at best 12mm thick and I doubt it would be of a structural grade. The steel cross members are spaced out at 500c/c or more. :nono:
If it was the floor of a house the joists would be at 450c/c max with a minimum 19mm thick structural sheet flooring in place.

The only way to fix the problem would be to increase the thickness of the ply or close up the cross member spacings.
Having said all of the above......ply is designed to flex and it is not really taking much in the way of load.
It may be flexing under foot however I can't see it failing unless it rots.

I have painted the underside of my ply floor with a few coats of Floods Penetrol. Can't really see the black paint that Jayco use providing much in the way of protection from the elements.