Yes, you now own a "lemon". The floor structure of 1/8" plywood / Styrofoam / 1/8" plywood is bonded together as the main base to build the rest of the trailer. The frame the trailer is built on is it a "I" beam or is it what is called a "Euro" type that is pressed steel with 3" holes along the side.
Yes, if you do rebuild the floor going to a 3/4" plywood is the way to go. Add a spacer so you make up the difference of the old bonded thickness of the old floor system. But your going to have to make sure you catch the load of the walls because the old floor system relied on the sandwich of 1/8' plywood / Styrofoam / 1/8" to support the walls.
My 2005 hybrid
had the euro type frame and the same sandwich floor system. My newer 2013 hybrid
has a "I" beam frame and the same sandwich floor system. But the 2013's floor is more of a solid feeling when walking on it.
Your outside walls are most likely the same sandwich thing plus a added inside decorative wall board. Here is a online link to a pdf file of the 2005 brochure of my Aerolite. On page 6 it shows a picture of the euro frame and page 7 a good diagram of the floor, wall and roof construction. You can see there is very little support that the euro frame offers. The main building blocks for that type of trailer is that "sandwich" floor.....otherwise that frame would twist to pieces. pdf 2005 Aerolite brochure
Is the styorfoam inside the walls glued to the outskirt of the camper?
The one thing thats been on my mind is if I pull the styrofoam ceiling down from inside the trailer would that "normally" damage the roof above and cause leaks?
Here you might be OK to pull the Strofoam down because I believe the roof was not a sandwich system like the floor and walls are. You said it was a 99' trailer, so I believe they were building them that way then as like today. One good thing is that you said it seems the roof is not leaking....That's good...