In our 396wp, I added 5000lbs recessed d rings.
--Located where I wanted the d rings inside the unit
--Measured from the side and end as to the location
--took the measurements underneath the rig
--made a small cut in the underbody cover
--observed what supporting structure was near the area I wanted to install the d rings
--was able to adjust the location of d rings so that I was able to drill two of the install holes through the hauler floor plus through a supporting 2x4 support.
--the other two holes of the d ring was drilled through the hauler floor with additional backing material
--d rings were through bolted not secured with wood screws
Also while I was up underneath, I installed wheel chocks with threaded bolts affixed on the under side of the hauler floor. This way, I can drop bolts through the chock and secure through the hauler floor. When the chocks are not needed, I removed the bolts, place the chock in the cabinet. The threaded nuts remain intact under the floor.
Works great for our Goldwing 1800, golf cart, kayaks or whatever else we need to secure. As it must be secure, so that our sliding glass door does not get damaged.