I had a similar situation and found that during a rainstorm the water would flow down the front of the trailer and leak into the front edge of the membrane, where it wasn't sealed properly. The first thing I did was drill a 1/4" hole at the lowest point in the sag of the membrane, to eliminate any pooling water. I later applied sealant to the leading membrane edge.
After that no more trapped water! I have observed membranes sagging on other trailers and believe this is no big deal..
When drilling the hole go just deep enough to penetrate the membrane. You don't want to drill thru anything that might be on the other side of the membrane!
Pat & Barb
2019 5.3L ECOTEC3 V8 Chevy Silverado 1500
2016 Aspen Trail 1900RB