If you lower the trailer and the crack presses itself back together on a warmer day I would mix up some epoxy and jack it up slather it in there and lower it... then add a few layers of epoxy and cloth to the inside to reinforce it. Obviously prepping the area first for it. Fiberglass is easily fixed.
My last sailboat had several cracks in the fiberglass keel. I ground the areas out let the cement balast dry all summer and layed up several layers of fiberglass cloth and epoxy over the keel and she spent 2 years in the water...got hauled from Hurracaine Sandy and the upper part of my boat was devastated, but what I repaired was still looking great and was keeping the water out.
All that is gold does not glitter, Not all those who wander are lost; The old that is strong does not wither, Deep roots are not reached by the frost.... J.R.R. Tolkien