I have a 1997 36' Dolphin got it new in 1997.
I remember getting under the back when I installed a back up beeper and at that time their was a sheet of fiberglass covering the opening, but I think it was put their to keep the water out that would spray up into the area if and when you were driving in the rain.
I never put it back.
I have a question for you, do you have a tandem wheel in the back and if so it has it's own break system try to check the master cylinder to make sure it has plenty of break fluid.
I for got about it and it cost me around $2,300.00 to have it repaired.
I don't know how long you have had your M.H.!!
Just a thought.