This is a travel trailer, not a 5th wheel correct?
If so, what I found on mine may not be applicable.
But what I have found at least with the 5th wheel TH's is, water - probably from the road gets up above the J wrap, which on the 5th wheel toy haulers goes all the way to the frame and the edge is then siliconed to the frame web.
Above the J wrap is OSB, which is the base for our toolbox section or basement floor.
The OSB is carpeted and between the carpet and wood it is like a sponge that has no airflow, so it never dries out.
I would look for the water intrusion before cutting any vents. I really don't think you can cut a vent big enough to get the air flow you need.