They could be rusted down and the slot is gone or filled with sealant. The last one I replaced that was the case. I bought stainless steel screws and put them in by hand when replacing. I went overboard sealing it by putting a bead of the caulk-tape like you use to mount a window underneath, then a bead of Dicor caulk over all of the screws and around the edge and followed up with Eternabond tape going all the way around to make sure it was sealed.
We are thinking of ordering our next RV new and having them leave the skylight out as it is nothing but cheap plastic that gets brittle and cracks causing another potential leak.