Originally Posted by candrews768
The issue is I can see from inside the trailer to where the access panel is. So I guess I am going to use insulation and flex seal.
I wouldn't recommend doing anything which will make it difficult or impossible to get into in the future. If there was need to enter that area once, there likely will be again in the future.
If you're intent on sealing it just get some adhesive foam weatherstripping and put it between the two layers.
Just because you can see from inside the trailer to where the access panel is doesn't mean anything, and there's little to no chance of water getting inside the trailer through that access panel, waterproofed or not.
Our trailers are not hermetically sealed underneath, and they're not intended to be.