Looks like you have to start reading from the bottom message. Sorry I am so long winded.
The bottom of the shell on my trailer was sagging so I raised it up with shims (1x6) between the metal frame and the shell. I then attached the plywood to the floor with the resin, hoping the two would bond. Unfortunately, this is one part of my reno that I am lnot happy with. I was virtually a complete novice at that point and did not do a great job of the floor. When I took the shims out, the shell sagged back down and so those shims remain in place. But if the shell and floor had bonded, there would have been a gap between them and the metal frame, I am sure. I do not know what I would have done at that point and I do not know why that gap would be there or what was done originally. Have you asked other Ventura owners about this? I hope this helps. I think that you have some other messages for me. I will get to those now.
I found some rubber feet for chairs that I inverted and stuck into the holes to prevent creatures or water getting in from below. Whenever there is a water event, I check the troughs and open the drains if necessary. I also made holes in the tops of 2 of the stoppers through which I ran any wires for the electrical that needed to go to the outside. So far this has all worked well.
As for the plywood, I used 5/8 inch marine plywood that I sealed with a coat of resin, especially on the edges.
I am sorry that I have not responded earlier. I was traveling for all of September and had had very little internet during that time.I was staying in Ontario provincial parks which are lovely but without wifi.
So, to your questions about the holes and the floor. I hope these answers do not come too late for you. There were a lot of holes in the shell at the bottom of my trailer so , like you, I had really no idea which were "official" holes ad which were not. I filled most of them but retained 5 or 6. There were several small ones by the door. I kept those because the floor and shell slant down toward them and I wanted them for drainage. Similarly, I have 3 holes in what I call the "troughs" on each of the sides. In fact, I enlarged those holes and made sure they were as close to the lowest points in the "trough" as possible so that any water that found its way in there would drain.
This message is running too long so I will begin a new one.