Picture #5 does look like it might have had a cup for a snap cap under it. The outer cap takes the beating from the sun and rain, will need to be popped off from time to time as they get that chalky look of old white plastic patio chairs. But the cup under it provides a bit of a seal (I added a wafer of butyl tape too) and does not degrade as fast since the outer cap is protecting it.
look like someone replaced rivets with screws. Might be stainless steel judging from the lack of rust and corrosion. Notice the screw inside the snap on the window frame is very rusted. Aluminum and steel react with each other and accelerate corrosion (rust/oxidation).
Picture #4 That metal snap on the window frame is like the ones that hold vinyl covers on the back of pickup trucks. Could also be used for a strap or holder or anything else that could be attached with a snap. If you have an awning
rail I might suspect a bag awning
Picture #2 looks like something was spilled or spread there, maybe to seal the rivet that was leaking?
Rivets vs. Screws debate has been going on for a long, long time. Some claim the Neanderthals were on the losing side of that debate in Europe but the evidence is not conclusive so for now it is just a theory.
I have a 77 scamp
and there was no sign of snap caps ever being used, and I know enough of the back history of it to make me think they were never installed. However they are now. I figured as I replaced all the rivets I could do it the "newer" way and probably gain an additional water barrier.
There are places where it is just infinitely easier to use blind (pop) rivets. They are intended to be installed from one side into a space you are "blind" to. You don't need to be able to see, or reach the other side to put a nut on, this alone makes them useful. Want to take your couch out to work on the wiring and floor? Couch attaches to the kitchen cabinet with rivets, short of taking the fridge
out you have to use rivets to make that connection.
My feeling on rivets is that in those iconic pictures of the guys building the Empire State building and other sky scrapers the round things in the beams holding it all together are rivets, different style & material but... rivets. I removed and replaced rivets still doing their job after 35 years so I guess they work. Not the only solution of course, and my curtain rod brackets are attached with machine screws so I could use small brass acorn nuts that matched the brackets.
probably leak, it would be a little surprising if after this long the butyl tape they are bedded into as a seal between the window and the FG shell was still sealed. On the other hand those screws might mean someone already did that job. Look at the where the wall joins the floor around the edges. Most leaks
the water ends up there eventually. Water runs down wall / furnishings then collects at the low point at the edge. Or at back side of wheel wells. Depends on if trailer is tilted nose up or nose down, and any left to right slope.