Our last three new fifth wheels all have had this same issue. Basically rain coming off the tires at 60 mph, is being blown past the coroplast covering on the under side of the coach. The coroplast covering is just that, a cover... not a weather proof seal. I had to use a 1/4 or larger drill bit and by hand, put a hole in that coroplast in the low point spots... to let the water drain. Obviously be careful to not drill into anything in the under belly, hence the using the bit by hand instead of a drill to put drains in the material. Its gotten to the point with us, that we refuse to pull the RV in rain.
Also FYI, that water in the under belly setting there will create a very moist climate under the floor of the RV. It will cause mold to start to grow. I ended up slitting the coroplast in two spots and putting large fans blowing up in the belly during dry periods to eliminate that moisture. Our landmark had bat insulation in the belly area, and it was soaked and weighed a ton. It had to be ripped out and discarded due to the water. It by itself would never dry out.