Removing underbelly material, what might I be getting into?

Had a similar problem and had to remove the plastic sheet under trailer. While there I took pictures and put copies in manual. Repaired all the valve seals and replaced sheet from a sign shop (4X8) $20. I cut access holes to change seals again if I have to or to make electrical repairs. I used new screws and large body washers. It was a job, not hard and glad I did.