I installed Joey beds in our 1993 U240.
What I found was the thin layer of fiberglass on the bottom (road side), a core of foam insulation, the thin layer of fiberglass on the basement side and then the fabric/carpet.
If the thru bolts securing them to the basement floor were in a beam or even better, relieved and thru a large wood plug epoxied in place and thru bolted using a large fender washer below, they will take a LOT more stress than those that are just secured through the two thin layers of FG and the foam floor core.
I have seen many installed just through the FG and foam. From under the coach it is easy to see if the bolts have wallowed out the hole. I redid mine with epoxied wood plugs (2" diameter if I remember correctly), as they were originally just installed through FG and foam.
The bolts really had nothing substantial to keep them in place, and and the weight of the tray as it hit its stops caused the bolts to wallow out the holes. This permitted the joey bed frame to move a slight bit in/out and worse, allow water migration into the core from the outside through the wallowed out bottom hole in the FG.
To reinforce this to take the side load of slide hitting its stops when opening/closing, I used a 2" hole saw and cut from the basement through the fabric, basement side fiberglass and into the foam core. Then removed the foam core with a knife (leaving just the center hole through the lower fiberglass panel).
Used the a 2 1/8" hole saw to cut wood plugs the same thickness as the foam core. Cut 4" X 4" aluminum backing plates to go UNDER the coach.
Cut 4" X 4" aluminum backing plates to go UNDER the coach.
So the Joey bed is secured by bolting the Joey bed frame through the solid wood plug, then through the aluminum backing plate. No problems in 10 years of use.