Moist spots are present under our 1980s trailer, but it seems to be fairly random. Help?

We've been smelling mold for a while now and there are a few soft spots in the floor. There was a leak in the kitchen sink but we apparently got that fixed. (I say "apparently" because my mother claims that the plumber went under the trailer to check, but who knows if he actually did.) There have been signs of some issues under the trailer, such as spongy flooring in certain spots in the kitchen and entryway. However, I looked under the trailer just now after someone else did yesterday (and left the siding open to let it "air out"--it's still slightly damp), and though I didn't get a great look, it didn't seem particularly moist under where the soft spots are. Just equally as most of the rest of the trailer from what I could see. The soil is a mix of dry and slightly damp spots. It doesn't seem particularly related to pipes, though I could be mistaken.

Help. What do we do? We don't have a lot of money to fix this.