I just went thru this same kind of issue. My water ingress though, was from driving in a storm, and the water blew into the under belly from the wet roads. There is a layer of batt type insulation in the center of the belly cavity that unfortunately holds water like a soaked towel. Drilling 1/4 inch holes (by hand..no power tools..your water tanks are there too!) will let the lions share of the water out, especially where sagging.
I ended up cutting access portals in the several places in the coroplast belly and removed the wet insulation. Did not re-install the batt insulation. Bought one of those high powered industrial blowers, that look like a squirrel cage design...and put the nozzle of that up in the coroplast, for weeks at a time on days where it would promote drying. It did dry everything out eventually. I just didnt want to drop the coroplast this year.