I then installed new insulation in the belly area. I had hoped to have the belly professionally sprayed with closed cell foam. This would have not only given me great R value but also helped seal the belly. However since I was doing the floor in parts, the cost to have it sprayed was prohibitive. This is because the setup cost is the same regardless of the spray area and there would have been multiple spray sessions. Too spendy.
I also did not chose to use foam board because the belly is curved so using flat board would leave some gaps no matter how careful I was. So I opted to use regular insulation, four inches thick.
Here is the front area. The floor is 3/4 inch marine grade plywood treated with multiple coats of CPES, a two part epoxy from The Rot Doctor http://www.rotdoctor.com. This treatment makes the wood basically impervious to rot. The epoxy encapsulates the cellulose in the wood so fungi and bacteria can't eat it, therefore no rot. After 3 or 4 coats the wood is saturated with epoxy. making it waterproof according to the manufacturer. That floor is going to last a long time.
Notice how nice the frame looks after the Corroseal treatment.
I inserted the plywood sections between the C channel and the frame. In some areas it was a tight fit to get the plywood in, so I used a little hammer persuasion and some crow bars to spread the gap. In other areas the plywood slipped right in.
In the front area shown above, the tongue wiring (battery and trailer lights) comes in through the belly so I notched the plywood leading edge as needed. Then I reattached the floor to the C channel with stainless screws using the original C channel holes and attached the plywood to the frame using self tapping screws.
Once the edge floor pieces were all secure then I dropped in the larger central pieces and attached those to the frame.
Solid!